Why he didn't mention he has a girlfriend?

I met this guy at my part time job today. I wasn't attracted to him at first but then when i got to know him a little bit better-he is very funny and he makes jokes and he is picking me- it felt really weird, good weird. instanttly i felt really attracted to him. I think he likes me too but i am not sure. He was staring at me a lot and smiling. Then when we were alone at the bus going home we were randomly chatting and he explained to me how a friend (F) of his once wore colourful leggings and a grey blouse. He said FRIEND! but then while i was stalking his fb profile obvsly :P i saw that he is in a relationship and then i saw a picture of his girlfriend wearing colourful leggings and grey blouse o. o
i am 99.999% sure the "Friend" he was reffering was actually his girlfriend.
What am I supposed to think?
Why he didn't mention he has a girlfriend?
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