Actually this is not limited to size 96. All of the sizes have different weight allowances. I believe this is because AMS2431/4 grade material is conditioned more than J441.

Therefore, AMS grade material will be slightly smaller thereby weigh less than J441.

AWS96 3.45 to 4.25 = 0.8 gram tolerance.
SCW96 3.800 to 4.700 = 0.9 gram tolerance.