I think that there must be a set of statistics that truly (honestly, objectively) represents what each individual kobold or frost giant is within the game world.
hmm...I can't say categorically that no game system and no set of stats could do that, but I will say that, AFAICT, the D&D mechanics are not and have never been a system capable of doing that.
Especially in the older editions its obvious that stats existed to function within the tactical/combat game environment and not to actually represent anything "objectively" about the creature in question in the game world. (Heck, most monsters weren't anything mechanically
but a pile of combat stats until 3e). 3e probably represents the zenith of full/complete mechanical representation as design goal, but even that edition maintains the same fundamentally abstract mechanical architecture.
After all, everything in the stat block can be measured objectively.
Actually, very few things in stat blocks can be measured objectively, AFAICT. Precisely which things varies a bit from edition to edition, but generally most D&D stats focus on combat, which D&D explicitly handles abstractly (most prevalently in older editions). That abstraction makes it next to impossible to actually perform the statistical reverse-engineering from
within the fiction. In particular the lack of correspondence between a game-world physical strike and the game-mechanical hit, and basically any and every implementation/definition of HP that I've seen make it particularly difficult.
It is a true fact that X specific creature will have Y% chance of hitting a stationary target, using a defined weapon at a given range, as corresponds perfectly to its ranged attack bonus.
Except that "hitting" in the game-mechanical sense of "do HP damage to" can only recently be associated with a singular attempt to swing/shoot/etc. the target, and even that isn't explicit in the rules. Certainly fighters didn't make one actual-fictional swing per round back when rounds were
a full minute long. Furthermore, its not evident that a "hit" at the table corresponds to your weapon actually physically damaging a creature (missile attack or melee) or whether they just spent a bit of luck, divine providence, etc. to avoid your efforts in this last minute. Thus, a creature's attack bonus may or may not actually correspond to their ability to physically strike a target in the scenario you describe. Even though that is the general assumption that we all make, its not actually evident from the way the rules work and how they are described (again with some variation between editions.)
ASIDE: How that applies and works (or should apply and work) for ammunition, ROF, and ranged attacks has been a fertile ground for argument and rules tinkering for as long as I've been discussing the game. Its only recently, with the shift to much shorter combat rounds in the WotC editions, that I've seen
that calm down.
It is a true fact that X specific creature will have Z% chance of remaining unaffected when exposed to a particular disease, as corresponds perfectly to its Fortitude (or whatever).
IIRC, even Gygax talks about poison saves representing a dodge to the poisonous blow, rather than just toughing it out. That is to say, a "save" might include both avoiding the exposure and/or resisting the disease. So, did a hero avoid the disease because his immune system fought it off or because he happened to do a better job cleaning out that wound today or because he happened to turn the blow so that the disease didn't reach the bloodstream? Which happened in any particular case is generally impossible to tell from the rules alone.
Prior to 4E, every metric in the game could be measured and determined empirically in game because it was a truth within that reality.
There are plenty of corner cases that make that not so. One example that springs to mind is "massive damage" rules and sneak attacks, critical hits, or backstabs, which would trump/shortcircuit one's ability to measure HP by checking how many licks with this broadsword does it take to get to the center of that ogre. Look up Schrodinger's Wounds for Pete's sake. I've even heard (well, "read") it argued by OSR fans on this board that Old-School casters aren't even aware that spells come in slots or that the slots have different levels or even what their spells actually
are! I don't personally see how that can be the case, but I've been told it solves no end of fictional inconsistencies regarding the question of "what just actually happened". ::shrug::
Simply put, the D&D (especially older-edition) rules function at such a level of gamist abstraction that it often requires our willful ignorance to relate them to fictional positioning. Indeed, the most common advice for dealing with all the inconsistencies that HP mechanics create is simply "You're thinking about it too hard. Just ignore it and go with the flow."