He doesn't actually want to say anything, is the issue here. Not until she speaks, anyway, and even then he makes a concentrated decision instead of flowing into it naturally.

"The only consistency of magic is that it is indefinable," he says, voice low. Severus is used to people listening to him and paying close attention, even if he doesn't consciously expect it. "I'm unsurprised that different worlds have different wizards and witches."