The worst injury John McClane actually takes in Die Hard is running over broken glass. Bullets don't bounce off him - they just get too close for comfort rather than hitting him.
My fault for a bad example, then. I could have sworn that I'd seen him take a direct hit, but he was mostly fine because he was wearing a bulletproof vest.
In Gygax's DMG, in the section on NPCs (that has the tables for generating personality traits etc) there are rules for stat requirements for NPC members of character classes. And they are different from the rules that govern PCs. Generally, they are more forgiving (eg a fighter does not have a min 9 STR, but rather after rolling stats for a fighter NPC you add 2 to STR; rangers, druids and monks also have noticeably less strict minima).
I was entirely unaware of that! I wonder how much of that design was because he expected players to use a more generous stat-rolling method, while NPCs were expected to use honest rolls. There's not a ton of difference between allocating (4d6, drop lowest) to your main stat, or just rolling 3d6 and adding +2 to your prime stat.
I think it's pretty fascinating that we've moved from "I want my game rules to model the world as much as possible" to "The game rules are the physical and metaphysical rules of the world, and directly observable to imaginary inhabitants even when you get results that sound nonsensical."
It's not that the game rules
are the physical laws of the world, so much as the rules
describe the physical laws of the world. While there's a lot of overlap there - you can throw someone down a 10-foot cliff with reasonable certainty that they aren't going to die from it, but that they will sustain some degree of injury, and you could easily kill someone if you do it too many times in a row - there are also differences.
The major thing is that the game rules are all written with certain assumptions, which allow them to simplify complex physics into something that works well enough for most situations. The falling rules, for example, assume a man-sized object. (All of the rules, really, are written for a human scale.) Blind adherence to RAW would say that you can kill virtually anything short of an elephant by throwing it from a 900-foot cliff for ~70 damage. The DM acts as a kind of oversight, though. If you throw a house cat from that cliff, it will probably
not die (though it will certainly be worse for wear); if you throw an elephant off that cliff, it probably
will die, in spite of what the rules say about terminal velocity. The simplifying assumption of a man-sized falling body no longer applies.
In a similar case, you may wish to alter the rules for stabbing people who aren't wearing any armor, in the rare situation when that assumption doesn't hold.
That isn't to say that the actual, modified rules for throwing an elephant off of a cliff represent something unknowable to anyone within the game world, of course. Merely that, whatever those rules actually are need not conform to what is written in the book. There is nothing in the game rules, RAW or RAI, published or house-ruled, that is
unknowable to science.