"Master Kenobi introduced himself to me as Ben. When I told Luke about your existence he connected the dots."

She's not really supposed to call him Master Kenobi, either, but it's difficult when speaking of someone you'd never dreamed you'd get to speak to. Besides, she wants him to teach her Jedi things, and maybe if she keeps at it he'll relent and take her on as a student in a more official capacity. Not that she's going to say any of that to Kylo.