Why she seemed chocked that a girl kissed my cheek?

This girl and I flirted a lot we both enjoy spending time at college. We both know that we are into girls. She is with another girl thats why I didnt do anything. I was with a girl who is straight. I took her to the train she was travelling. She thanked me to come along and hugged me and kissed me on the cheek. She didnt mean anything. However this lesbian girl i saw her suddenly out of no where at the same station traveling too and she seemed shocked i never seen this look on her face before. She waved i didnt know what to do. So i said goodbye to my freind i was suprised too and didnt know its her until i left. Why she seemed shocked if she is already seeing another girl? I know we both like each other but we never went out or anything???? Need opinions?