Ok, here's a theory...
In some pic of KH2, BHK was ready to battle Axel, saying that he won't allow Axel to meet Sora.
However. In KH: CoM, Axel do meet and battle Sora, TWICE!
So here's the thought...
Appearantly, BHK most have been a Keyblade Master BEFORE Sora, probably a LONG time...