How did they arrive at each of the shear values listed in the UBC?
Plywood diaphragm research preceeded plywood shear wall research, so many of the formulas used to determine shear wall strength were adapted from the eariler research. Here's an example from the American Plywood Association's Research Report #154, Structural Panel Shear Walls, revised in May, 1993 and available from APA  The Engineered Wood Association (206) 5656600 (ask for publications):
(beginning of text)
APPENDIX B
The building codes allow calculation of diaphragm and shear wall values using the principles of mechanics. Such a calculation involves several factors not shown in the building codes, such as influence of framing lumber width and panel thickness versus nail size. This appendix lists these factors and details the steps required to calculate design shears.
The currently accepted shear wall design values were based on applying a load factor to ultimate loads from tests of actual shear walls. The relationship between the design shears for different nail sizes is based on the relative lateral design values for the nails in the Uniform Building Code at the time the basic shear wall research was conducted. Lateral nail values and their relationships were changed in the 1964 Code; however, the original tabulated values based on tests were never adjusted to accommodate these changes or to make them correlate with the new individual nail design values. As a result, computation of shear wall design loads using currently accepted nail values and the design factors listed in this appendix will give conservative results.
Previous tests of fasteners, shear walls and diaphragms have established the following factors to be used in the calculation of design shears.
Example No. 1
8d common nails, 3".o.c., 3/8" APA Rated Sheathing, parallel to 2x Douglas FirLarch framing 24" o.c.
76 x 1.1 x 1.6 x 0.89 x 0.90 x 4 x 0.83 = 356
plf
Use 355 plf (Recommended shear value in building code is 410 plf)
Example No. 2
10d common nails, 3" o.c., 15/32" APA Rated Sheathing, 3" Douglas firLarch framing 24" o.c.
90 x 1.1 x 1.6 x 0.90 x 4 = 570plf
Use 570 plf (Recommended shear value in building
code is 600 plf)
(end of text)
Example No. 3
For comparison, let's redo the same examples using uncoated
box nails as fasteners...
Nail Design Values for Single Shear Connections  Combined Table  
(Both members of identical species)  
Side Member 
Nail 
Nail Diameter 
Penny 
G=0.55 
G=0.50 
G=0.42 

t_{s}  L  Common 
Box  Common 
Box 
Common 
Box 
Common 
Box 

0.5  2  0.113  0.099  6d  67  55  59  48  47  38 
2.5  0.131  0.113  8d  85  67  76  59  61  47  
3  0.148  0.128  10d  101  82  90  73  73  59  
3.25  0.148  0.128  12d  101  82  90  73  73  59  
3.5  0.162  0.135  16d  117  89  105  79  87  65  
4  0.192  0.148  20d  137  101  124  90  103  73  
4.5  0.207  0.148  30d  148  101  134  90  112  73  
5  0.225  0.162  40d  162  117  147  105  123  87  
5.5  0.244    50d  166  151  127  
6  0.263    60d  188  171  144 
Example No. 1
8d box nails, 3".o.c., 3/8" APA Rated Sheathing, parallel to 2x DouglasFir Larch framing 24" o.c.
59 x 1.1 x 1.6 x 0.89 x 0.90 x 4 x 0.83 = 276 plf as compared
with 355 plf using common nails.
Example No. 2
10d box nails, 3" o.c., 15/32" APA Rated Sheathing, 3" Douglas firLarch framing 24" o.c.
73 x 1.1 x 1.6 x 0.90 x 4 = 462 plf as compared with 570
plf using common nails.
Plywood Strength
For a point of reference, what is the strength of the
sheathing material versus the calculated shear based on nailing? You'll
need three bits of information:
Allowable Unit Stresses for Construction and Industrial Softwood Plywood  
(In pounds per square inch  normal loading)  
(To be used with section properties in Plywooddesign Specifications  See UBC Standard 232)  
Stress  Species Group of Face Ply  Exterior AA, AC, CC  Exterior AB, BB, BC, CC (PLUGGED)  All Other Grades of Interior Including CD Sheathing  
Structural CD (Use Group 1 Stresses)  
Structural II CD (Use Group 3 Stresses)  
Structural I AC, CC (Use Group 1 Stresses)  CD Sheathing (Exterior Glue)  
All Interior Grades with Exterior Glue  
Wet  Dry  Wet  Dry  Dry  
1. Extreme fiber stress
in bending (Fb) Tension in plane of plies (Ft) Face grain parallel or perpendicular to span (at 45 degrees to face grain use (Ft)/6) 
1  1,430  2,000  1,190  1,650  1,650 
2,3  980  1,400  820  1,200  1,200  
4  940  1,330  780  1,110  1,110  
2. Compression in
plane of plies (Fc) Parallel or perpendicular to face grain (at 45 degrees to face grain use (Fc)/3) 
1  970  1,640  900  1,540  1,540 
2  730  1,200  680  1,100  1,100  
3  610  1,060  580  990  990  
4  610  1,000  580  950  950  
3. Shear in plane
perpendicular to plies (Fv) Parallel or perpendicular to face grain (at 45 degrees to face grain use 2*Fv) 
1  155  190  155  190  160 
2,3  120  140  120  140 
120  
4  110  130  110  130  115  
4.
Shear, rolling, in the plane of plies Parallel or perpendicular to face grain (at 45 degrees to face grain use (4/3)*Fs) 
Marine and Structural I  63  75  63  75  
Structural II  49  56  49  56  
All Others  44  53  44  53  48  
5. Bearing (on face) Perpendicular to plane of plies 
1  210  340  210  340  340 
2,3  135  210  135  210  210  
4  105  160  105  160  160  
6. Modulus of
elasticity In bending in plane of plies Face grain parallel or perpendicular to span 
1  1,500,000  1,800,000  1,500,000  1,800,000  1,800,000 
2  1,300,000  1,500,000  1,300,000  1,500,000  1,500,000  
3  1,100,000  1,200,000  1,100,000  1,200,000  1,200,000  
4  900,000  1,000,000  900,000  1,000,000  1,000,000 
Given the descriptions, we know were are not dealing with Structural I panels. We also cannot be certain we are working with only Group 1 species...so we'll assume Group 2 stresses (and verify with our supplier). No notation is made about how these panels were manufactured, so we'll have to look at all possible combinations.
Allowable Shear = (Effective Thickness in.) * (Shear In Plane, Fv) * (12 inches/foot) * (1.33 seismic increase)
3/8" APA Rated Sheathing
3/8" Unsanded Panel, Shear (lbs./ft.) = (0.278) * (140) * (12 inches/foot)
* 1.33 = 622 plf versus 355 plf
3/8" Sanded Panel, Shear (lbs./ft.) = (0.288) * (140) * (12 inches/foot)
* 1.33 = 645 plf versus 355 plf
15/32" APA Rated Sheathing
15/32" Unsanded Panel, Shear (lbs./ft.) = (0.298) * (140) * (12 inches/foot)
* 1.33 = 665 plf versus 570 plf
15/32" Sanded Panel, Shear (lbs./ft.) = (0.421) * (140) * (12 inches/foot)
* 1.33 = 943 plf versus 570 plf
Of note is the disparity between the answers for 15/32" panels. One provides a 16 percent margin above the nailing. The other provides a 65 percent margin. Chose the panel that best suits your needs while providing the maximum overcapacity at the same cost.