You don't know me, cos you only choose to see a version of me. That's why it surprises you too much when you finally discover who I really am.
The second reaction is much truer. This is because -most of the time- when we "get to know" someone, we don't actually see the person as who he really is. As a creature with strong imaginations, we see people the way we want to see them. This gets worse we're "in love" with someone (and so much worse when 'we' are girls). The colours of love sure are more bright and saturated. This is why there are a lot of nice people end up with people who don't deserve them.
It's easier to love an imagination...
It's not always a bad thing though, mistaken someone for who he isn't. There scenarios go both ways. There'll be a time when you think someone is not bad person, but she turns out to be a "tragically misunderstood character". When you thought someone is lazy, but really she's just suffering a ADD (Attention Deficit Disorder) and turns out to work great when she works alone. When you thought someone is weak and timid, and she's really just being polite and has been feeding your ego. And there'd be time when one day she decides she has enough and shows that she's actually strong and independent (and decides to flip you one).
In either case, it's always good to be honest to yourself, don't lie to yourself. As Syrio Forel wisely said, "See with your eyes." Don't see with your judgement or imagination. See things and people for what they really are. And when you've seen, don't be angry or bitter, just accept it and move on. People are not here to make your life miserable (you're not that important), they simply are.