It's the world's "mechanic" explained in the intro, every girl that's tamed calls the MC by the title "Master", but you can change that for each of them.
Why would they?
As I said, they both knew that she was hiding in the attic, and the master-tamer thing is part of the world, so it’s not something that’s a rare and weird occurrence.
I feel like something is getting lost in translation, because I’ve said this multiple times now, and I know it’s also said in the game.