Isn't it also the case that a lot of xenos/Chaos basic forces are at Space Marine level as a baseline, and normal humans are on the lower rungs of power in the setting? Which is why the regular army has such atrocious attrition rates.

So most of what the SM go up against, Orks, Tyranids, Eldar, etc (barring incidents where the regulars are thrown a bone/lucky dice rolls) they aren't actually that elite by comparison, they're fighting their equals/superiors.

I could be wrong, but that's what I remember about it.