An unexpected change of language

I went to a training session today, supposed to make my A-level teaching match the board specifications better, hence allowing for better exam results. As I arrived, the woman delivering the session was walking around registering participants and chatting a bit here and there. From where I sat, I could hear her having an animated conversation in French with one of the participants, which was natural given that she was French, as I deduced from her name written on the board next to the session's timings. After a while she approached me, we went through greetings and a bit of small talk in French, and then suddenly she switched to English - no reason, no warning - and stuck to English even though I stuck to French. And then it dawned on me - the change was not that inexplicable. It coincided with me indicating my name on her register. My name is rather obviously not French...