[voice]
[ that's...not exactly an answer, but perhaps it was as he'd thought, and it was different in the future--a courtesy title even? But if some considered her royalty, then Merlin probably should, too. She seems nice, and he doesn't want to offend her he only does that to the prat-ish royalty, after all ]
Oh, I guess, uh, I'm sorry, My Lady.
[ and because, he might be apologising for his lack of respect in addressing her so casually, and even raising his voice, earlier; but that doesn't mean he can't still be cheekily curious about what she has said ] What would those people say if asked, then?