I am fairly new to the 5th edition, I am back to RPGs after a 15 year hiatus, and I might be ignorant of what you are talking about. But I have not seen gross inconsistencies in how the rules are presented to indicate there is a difference in how the world works for PCs and NPCs. A human fighter with a vorpal blade chops your head off with a critical hit whether he is a NPC or PC. A Balor does triple damage on a critical hit, whether the target is a PC or NPC.
A Balor wields a vorpal longsword, which they don't bother to spell out for anyone because it might be confusing. The way a vorpal sword works in the DMG is that it just decapitates someone on a critical hit, unless they're too cool for that (i.e. they have legendary actions); if they're too cool, and just decapitating them instantly would be lame, then it does a bunch of extra damage instead.
The way the balor's sword works in the MM is that it just does a bunch of extra damage on a critical hit; the designers automatically assume that it's going to be fighting PCs, and decapitating them instantly would be lame, so they cut to the chase and go straight to the extra damage step. You can see
how they designed it, and that it
basically gets you to the same place, but they've tried to
hide the underlying logic by simplifying the game mechanics so players don't have to
think about it. Except for those players who are veterans of certain previous editions, who
expect the game mechanics to stay intact through the whole process, and are left questioning
why they would bother using two different mechanical subsystems to represent the same in-game reality.
A better example of the same principle can be found in the NPC listings in the back of the MM. I forget which NPC best demonstrates it - it was either like a bandit leader, or a gladiator or something - but you can clearly see that it's intended to dual-wield and has extra attacks. Instead of just using the normal dual-wielding rules from the PHB, and having the Extra Attack feature, it has the Multiattack action that you see in monsters: it can make two attacks with its main-hand weapon, and one attack with its off-hand weapon, as an action. It doesn't require a bonus action to make the off-hand attack (not that it has any other options for what to do with a bonus action), and it adds its full stat modifier to the damage roll. It's
basically a high-level fighter-type, but they present it using monster rules, which means it doesn't
quite line up exactly. But it
mostly works in practice, if you don't think about it too much.