Why do people still call me quiet/shy when I'm not?

Probably just do what you've been doing... I wouldn't worry about it. If you don't have anything to say (I find myself in that situation a lot), then you can't force yourself to say something. I am the same way, and I just tell people that I like to make my words count for something. To me, there's no use in saying things just to say them; words need to have meaning for me to speak them.