Bolter damage inconsistency across units

[xerrol nanoha]

There are lots of units that use bolters, but why is damage among them so inconsistent?

Devastator 12
Scout 13
plague marines 18
noise marines 18
chaos havocs 23
chaos marines 23
Tactical marines 25

Why for instance, does the devastator bolter do 12 damage while the havoc bolter does 23 damage?
why are chaos units' bolters so much stronger in general than space marines (except tacticals, but even so?)

Silly Xerrol, that makes it balanced!

I was more asking for the design thoughts, as apposed to comments like "just because" or "that's the way it is".

[Oddnerd]

It is just how the game is designed. The game does not have separate rolls to hit, rolls to wound, armour saves and so on like tabletop. The damage value should not be interpreted literally as the sole product of how hard a particular round of ammunition hits, it is a function of several things - weapon strength, unit skill, and of course, the need to balance.

A scout bolter doesn't do half the dps of a tac marine bolter because the developers want you to think scouts are carrying lower calibre bolters, it does half the dps because a scout with a bolter is presumably less experienced (and of course, because if you gave a scout squad the dps of a tac squad it would be super imba).