This report is one of the main reasons I have swapped down from .40 to 9mm:
Conversely the 9mm can probably be fired fastest of the common calibers and it had the most rounds fired to get an incapacitation (2.45). The .40 (2.36) and the .45 (2.08) split the difference. ... If a person takes an average of 5 seconds to stop after being hit, the defender who shoots a lighter recoiling gun can get more hits in that time period. It could be that fewer rounds would have stopped the attacker (given enough time) but the ability to fire more quickly resulted in more hits being put onto the attacker. It may not have anything to do with the stopping power of the round.
You can't shoot a fraction of a bullet; 2.45 rounds of 9mm and 2.36 rounds of .40 both round (ha!) up to 3 rounds. Might as well have 15 in the mag instead of 13, and get faster followup hits. Via An Alternate Look at Handgun Stopping Power.