Or in some cases, they are friendly to ya because they need you for something.
But yes, I got suckered by those girls who are friendly with everyone else, making it hard to distinguish them and girls that actually do have an interest in you. With these ones, they make us guys run around in circles sometimes.
The girl I had this exp with seems incapable of bitchiness but well she showed me after her indecisiveness on friend/stranger caused me to get paranoid and now tells me even the friendship is over. Yet she had the gall to ask me ever so nicely like nothing happened at work when she needed something from me. When I rebuked her afterwards she goes and say "it's unavoidable to not talk to me at work, just like when our senior told her to give me something (which she says she would die first if otherwise)".