What are your headcanons? Here are mine:
Ash is NOT 10, he's 14-15.
Flint (not Flint from Sinnoh, but Brock's dad) is the cause of Pikachu's...inconsistency.
Brock graduated sometime during Ash's Unova journey (he's that good :P )
I have more, but that's all I can think of from the top of my head.