I was thinking that maybe Mark doesn't so much think about how *he* will sound delivering the lines as he writes them, but they're just there in Mycroft's voice. I'm sure everyone's brain is wired differently, but sometimes when I read something, I don't so much "hear" the actor's voice in my head but more my own voice imagining to be the character. I'm also quite sure that, as an actor, you know how to separate yourself from the character you play.
But since I've never written anything that I would act out later myself, this is all just theory.