You don't have to say he doesn't know what he's talking about. You simply have to treat it as what is likely closest to the truth anyway. Likely NuWho hasn't varied his age because it's easier on the writers to just leave him at one age they all use. If that's the case than the explanation of the Doctor's age constantly being 900 is like a woman who's age is constantly 29. It isn't that she doesn't know her age. It's that she prefers to hold at that one age. I think the Doctor is well over a thousand years old and damn well knows it. He just has stopped admitting to his actual age.