it makes sense but seriously, after traveling through Kanto, Jhoto, Hoen and Sinnoh, when he flies to Unova with Oak and his mom, it is said he is still ten years old (and there was a Christmas special in both Kanto and Jhoto, so it has passed at least one year). My theory is that Ash was born in February 29 th, so he just consider that it's his birthday in four years (I know ten isn't multiple of four but it's the best explanation I could think of).