Why does my face ID work with someone else's?

Hi I wanted to ask why my face ID works on my mum's iPhone becuase she has set it up and my face works aswell on it like the phone unlocks and I wanted to know why? She hasn't set up the alternative face and when she was setting up the face ID on her phone I wasn't even near the camera or near her when she set it up so could someone please help me thank you