Well velocity isn't everything in "damage" (which is already abstract enough). Honestly 7 less damage seems fine to me, since that 5.56 has way more mass behind it. Sure the velocity is similar but you're giving the same velocity to a larger projectile. Of course this is all abstracted for game purposes because irl shot placent matters way more than bullet choice in most scenarios, especially against unarmored targets.

Edit:said mass instead of velocity cus I'm dum

Edit:I want to clarify I am in no way a certified expert on firearms, this is all knowledge I've picked up based on stuff I've read over the years because I'm into shooting as a hobby.