Why would a cute coworker tell me this?

I work with a girl who I find really attractive. Her and I joke around all the time and even flirt a lot of the times. There is some sort of tension between us. She is around me a lot usually or in my vicinity.
Sometimes when I tell her about my interactions with the female clients, she's usually like "omg she was hitting on you" or "she was totally flirting with you".
Why would a cute coworker tell me this?
Post Opinion