I always wondered how an entire faction of unmodified squishy humans who would die if sneezed at by pretty much any other unit in existence, suddenly acquire a strength on par with Power-Armored demigods and Orks just by being promoted to a Comissar or Commander. This is entirely judging by the computer games, I have no experience with the fluff or the miniature games.
Well, the computer games need to be balanced, y'know, and nobody really wants your commander to die from stray bullet. But there are examples of stuff happening just like that in Universe, and the most sane explanation is Emperor (or Tzzentch) helping them. Some of Guardsmen are so badass and awesome that they catch his attention (which, understandably, usually happens when they're noticed by superiors and get promoted) and he proceeds to keep them alive. Most Commissars get at least weak version of this as standard equipment, after all, they're orphans that spent their whole life praying at least half as hard as Sisters Of Battle. And Sisters Of Battle cause miracles on daily basis.
That... or insane batshit crazy amounts of pure luck.
I think it's also a symptom of something I can't exactly name, but a little theory I have about Universes where "Anyone Can Die!"(TM) where people lives are like so much used tissue paper... yet there is a group of people who consistently avoid death despite not being superhuman, that strains credibility. It's something I should probably talk about in the Pet Peeves/Movie Nitpicks thread tho.
Also Authority Equals Asskicking plays a big role in WH40k from what I can see.
Warhammer 40k is a universe where individuals are meaningless. Even people who "matter" only have power because some faceless faction or functional god gave it to them. Unfortunately this doesn't make for good gameplay OR good storytelling, so GW writers are constantly backtracking or ignoring their own rules.
Probably the most extreme example of this is Rogue Trader. Want to set a tabletop RPG in a universe where the individual is irrelevant and being a special snowflake is punishable by death? Its cool, just invent a new type of person that breaks every rule of your universe.
Rogue Traders:
Are allowed to interact with xenos and heretics without being killed
Don't have to necessarily take orders from a higher power and are basically immune to the imperial beaurocracy
Are allowed to engage primarily in economic actions without being serfs (THERE IS ONLY WAR, unless you're this particular person)
Have nothing to defend, no real stake in any of the galactic conflict, and are thus largely immune to the whole "genocided through no fault of their own" thing that basically everyone else is subject to.
Basically, they don't belong in the 40K universe but have to exist because a character that makes sense wouldn't be a good RPG character. Ditto for imperial guard badasses. Does that even make sense in this universe? No, no it does not. Not on a literal level or a thematic one. But it needs to happen for meta reasons.
Funnily enough the reverse is also true. For the Eldar and Space Marines to have survived as long as they have would need to be individually and as a group unparalleled badasses capable of easy victory facing odds of 100:1 or worse. But workable gameplay terms they need to be weaker than that by far. Both seem to trade about 1:4 with
imperial guardsmen, which are literally just ordinary humans handed guns that even the Imperium considers shitty. Eldar/SM also trade about 1:2 with orcs, which is just sad considering that orcs are arguably MORE expendable than humans (there's some silliness involving spores that makes orcs into omni-present pests). It gets even weirder when you consider the supposedly young race of the Tau does BETTER than either of them in gameplay terms, considering fire warriors are their IG/guardians equivalent and battlesuits are their terminator/aspect warrior equivalent. Going by the gameplay the SM and Eldar should have been wiped out in a single human generation. They simply aren't powerful enough for the odds they keep throwing themselves into; yet supposedly they've lasted unimaginably long.