Ooh, ooh, I think I know!
I think it's because an expanded 9mm bullet makes a bigger hole than one that isn't expanded.
But to make it expand and go between 12 and 18" into ballistic gel after going through 4 layers of denim, requires more velocity and therefore more recoil than with a bullet that doesn't expand.
But wait.....if a bigger hole is better than a smaller hole, and the cost for that is more recoil.... wouldn't that same concept apply to .40? A bigger hole than a 9mm, but it cost more recoil?
Wow! This is super confusing. I understand why some guys are having a hard time with it.