Because humans don't have a clue about anything. She might not know what she wants; she might be pretending to like you so as not to hurt your feelings or out of spite, she might be pretending not to like you because she feels sorry for you or out of spite; she might like you but get dispirited because she thinks you don't like her, at which point you try your hardest to show her you do like her so she perks up again, and you're happy about it and slightly more relaxed and so the cycle continues; she could be bonkers; she could just have a very busy lifestyle; she could have social or behavioural problems, or personal problems that affect her life. Or she could just be completely indecisive.
Dunno.