Well, the 124 gr bullet is 8% heavier than the 115 gr bullet. You were measuring velocity, not the cup pressure in the chamber or energy of the fired round. You may also want to review the weight differences in your reloading manuals for your powder - you will see that you top out pressure-wise with less powder for the 124 than you do with the 115. An even better example of energy difference between lighter vs heavier bullets (and without getting into a debate over 9mm vs 45 cal), you get alot more energy from a .45 acp at 950 fps than you do with a 9mm at 1200 fps - the major math-factor is the weight, not velocity.