What I've found to be more common is when girls act very interested on dates and after, then they go all weird all of a sudden. At any rate, the idea is the same. Girls commonly give off different signals at different times and never bother to give any explanation or reason. Really, it's girls who leave a trail of confusion. ;) So yeah, good question. Why IS that?
I THINK (and I can't speak for all girls) but often a girl is friendly, nice blah blah blah beFORE the date, because there is no pressure. You wanna be nice to everyone, you may like them as a friend don't wanna hurt their feeling, etc. Then you go on a date and there is (unwanted or not) a degree of pressure.Also, I speak for myself on this one, I don't have gobs of dating experience so sometimes the "stigma" of a date makes me nervous. Yes, I'm a dork. I know :)
When I am completely comfortable (meaning I only see you as a friend) I will be more friendly and outgoing with you. Backwards- I KNOW! When you ask me out I may act weird because I don't want to give you the wrong impression, but I believe in going on at least one date to give everyone a chance and to expand friendships. It is almost completely unconcious, but sometimes I turn off all charm completely as a defense mechanism. Weird. Does that make sense? Girls are probably just trying to be friendly and nice. That doesn't always mean they are interested. CONFUSING! Sorry. More times than not, when I am interested I shy away from that guy. Ridiculous. I have to like someone a ton in order to make moves.