I suspect, however, that one reason they are keeping it vague is simply to allow existing Realms fans to do their own cherry picking.
You don't have to suspect - they've straight-out said as much. That intent has been carried into the novels, where they're leaving a lot of stuff covered by a "fog of war". And unless they suddenly change their philosophy, it's going to stay that way. Ed has talked about how this actually reflects the setting - communication is far from perfect in the Realms, so various versions of a given story will be possible. And this is desirable in the game, because it opens up the possibility space for individual games.
If something does come along in a novel that contradicts your game, you have two options:
1. Say that it didn't happen like that in your version of the world (though potentially it's a story being told there).
2. Say that it did happen like that, and anything you've previously said that contradicts it was misreported.
These are especially easy when it comes to the doings of the gods and other powerful beings, as they will rarely be in play in a direct way.
Of course, if something happened directly to your PCs, then it should be the truth. Your game is a story where they are the heroes, after all. Even in the more lore-heavy Realms of the past, it has been very possible for the players to change the world in ways not reflected in the 'canon'. In fact it was probably a lot easier, unless the DM decided to impose heavy-handed restraints.
Ultimately, this is an RPG. The story should be about the experiences of the player characters. The lore should be whatever it needs to be to support that.