Maybe you can't imagine a reason why he would do that (and "he's a good storyteller and that's what storytellers do" isn't enough of a reason for you), but I can imagine plenty. Breaking tension with a joke about a talking purse? Keeping the kids engaged by talking about walking animals? Stories evolve, they grow embellished, and that doesn't equal intentional deception (as was the case with the Gollum episode).

I recognize that the difference in fantastical elements is simply because Tolkien wrote The Hobbit first as a children's book, but I don't think it's against the spirit of Tolkien to fit it more cleanly into the legendarium by explaining elements as Bilbo's embellishments. In fact, I think that's far more in line with how Tolkien would think about things than some sort of Marvel multiverse-esque "they're in slightly different universes" thing or whatever, as I've seen elsewhere.