Throughout all of DnD (although 4e is a bit of an exception to this) - the creators have very much relied on something termed the "Friendly Uncle" method of teaching the game.

That is, you don't really learn to play DnD by opening the rulebooks and reading them end-to-end, you join a campaign with someone experienced (the "friendly uncle"), and they teach you the game as you go. The books serve as a sort of "reference" for common rules and making ideas, but most of your roleplaying experience comes from others.

As such, most groups and players of DnD have lots of conventions and house rules that they may not even realize are house rules. They have explanations for things that aren't in the core books, and they have rules and explanations from older editions - which often defined things in more detail. So much of the game is received wisdom from other players, and derives from someone "making it up" a decade ago when they couldn't find or didn't know an answer.

In one regard this is pretty good, the game is a living entity, constantly evolving, and allowing players with vastly different preferences to enjoy what is (ostensibly) the "same" game.

In the other regard, it's really frustrating when it comes to actually pinning down the mechanics or understanding of anything.