Aqua is a Keyblade Master as well...as far as I know, the only other one present here. [There was that business with Riku, but he seems to be gone now...]

And in that case, I guess it is just a matter of timing. She hasn't been Master for too long, and considering she was formerly my apprentice, in my case I believe it's simply old habits dying hard.

The other Keybearers should address her as such, though.