Why do my questions end up in areas that are not requested.?

Assuming you mean your questions are being moved after you've placed them where you want them, it's because Yahoo has given Level 7 users the ability to move the misplaced questions of other users to different categories so they'll get better answers. While many L7s use this ability responsibly, some use it to move even properly placed questions into inappropriate categories, either because they don't like the asker, or the question, or just because they can. You can move your question back to where you want it by using the "edit category" option found in your edit button but there's no guarantee it'll stay there. I really wish Yahoo would spend more time thinking these ideas through before they implement them but unfortunately that seems unlikely.

It seems that Yahoo Answers tries to place your question into right area simply by making sort of guess about it using topic as keyword(s).

Are you sure your questions really belong to the area you are trying to place em into?