The ending of The BFG is the happily-ever-after type. The killer giants are put in a pit where they can’t eat humans anymore, and Sophie and the BFG get a sweet setup in neighboring houses. Plus a lot of gifts. But the ending also comes with an added twist: we find out that The BFG is (supposedly) the author of the book.
There’s no way we could have guessed this ahead of time. The narration has referred to the BFG in third person, and in dialogue, the BFG speaks his own brand of English. Or as he would call it, “wigglish.” So the ending is definitely a surprise.
Why the twist? The better question might be: why not? The twist is straight-up proof that the BFG has definitely improved his language. So there’s that.
But don’t worry, there’s more to it than that. It’s a fair guess that Dahl went with that ending to add yet another fun layer of magic to the story. The BFG has already surprised and impressed us with its inventive magic about a giant who understands music and hears dreams. The fact that he secretly wrote us his own story is like icing on the mystical cake.