AbdulAlhazred
Legend
Suppose the following scenario occurs while playing a game of D&D3.5.
The PCs meet a local baron who impresses them as being very competent: He can kill 4 kobolds with 4 different greatsword strokes in 6 seconds; he can regularly make a long jump of 30 feet; he can swim underwater for 1.5 minutes (or hold his breath motionless for 3 minutes); etc.
The DM tells the players that, while out riding one day, the baron fell off his horse and hit his head on a rock, which instantly killed the baron.
The players reason as follows: According to the player's handbook, for a human to make 4 attacks per round with a greatsword, he must be a fighter of at least 16th level. This is supported by his ability to regularly make DC 30 jump checks (i.e. ~ +4 Str plus 16 ranks in jump gives him a +20 modifier). If he can swim underwater for 90 seconds, the baron must have at least a 15 Con. Minimum hit points for a 16th level fighter with 15 Con is 48 hit points (with the unlikely assumption he rolled 1 every time).
Falling 10 feet off a horse does 1d6 damage. Perhaps the DM added another 1d6 for the rock. Maybe even assume that the fall did "critical damage" and doubled. That still means the fall did at most 24 hit points of damage, and could not have killed the baron.
Therefore, the players reason that that something else must have gone, and they go off in search of the baron's murderer.
As a DM, is your reaction:
A. The players are very clever. By knowing the way the game world works, they have discovered something important.
B. The world doesn't work that way. Hit points, character classes, etc. are constructs only to describe how the PCs interact with the world.
IMHO, for D&D3.5, the correct answer is A. The world really does operate according to a set of rules, even if those rules aren't always visible to the PCs. (Absent some bizzarre prestige class or something similar)
However, for D&D5e (and 4e), I would say the correct answer is B. It is entirely consistent for a baron to have all kinds of ad hoc special abilities (multi-attack, hold breath) that don't come packaged with high hit points.
Right, and my answer is that ALL OF THE RULES are just conveniences, conventions that exist so that we can get on with the story. They are 'real' to the extent that they model the in-game reality and they are not 'laws of nature' in any edition of the game. If the Duke fell of his horse and died, its just because its quite possible for a character to die that way. The DM would NOT end a PC that way (arbitrarily at any rate) by decreeing that he fell off a horse and died, but he'd certainly be in his rights to do so for an NPC. The Characters might question the story of the fall, reasoning that such an expert rider was unlikely to fall off a horse, and look for some nefarious reason for it. They might find one, or not. The reasoning is consistent with the rules, but not stemming from or based on them. I would call the sort of chain of reason you outline as meta-gaming and to be frowned on, though I think all players do it to some degree.
And I believe that Gygax, to the extent that we want to analyze his style and intent in writing the game, would agree with me. There are various places where he calls out meta-gaming and various times when he invokes realism and plot over rules. He certainly never intended a rule to get in the way of the fun.
So, maybe the answer is "the Duke is very athletic, but he's not 16th level, he's a 5HD stat block with some special sword fighting techniques as a special ability" If he fell and broke his head, that's maybe statistically unusual, but not impossible.