Aiyah..

It just means that the girl's not that self confident lah..
Say, you're a guy, right? You're in the MRT, standing cos there aren't enough seats. You notice that a girl (not that pretty, but cute) sitting nearby is looking at you.
What do you do?
The average guy's reaction would be to suck in his stomach, flex his shoulders and maybe stroke his hair backwards or whatever his hairstyle is, making sure his hair is in place and not messy. You know that nothing would come out of it (with the exception that you approach the girl and ask for her number, lah), but still you do it. I do it too.
It's called EGO. Yup, girls have that too, you know... but they don't suck in their stomachs and flex their shoulders. Usually the just stroke their hair, maybe sit up straight, correct their posture, show off their assets. Most girls love to know that they are desirable..
Well, I speak from experience, so if my experience is wrong, do tell me, awrigh?