I can not say exactly how common it is for girls to remove hijabs going to Singapore since like you, I am not on every flight. I can say that it has been my experience on the flights I have taken. It was my wife who first brought it to my attention.
I would have to guess that anything reported on as to how many females feel they wear hijab due to outside pressures would be in that same report you have saying there are no pressures. Care to reference that? Sorry, my housekeeper while never wearing a hijab while working does to and from, while meeting with school personnel, and when she thinks it is safer for her as a woman.
I get my information from ones I speak to directly. There are no better sources than those who hold their own feelings on things. If someone tells me they wear hijab because it is what they want, all the power to i them. If soneone tells me they wear a hijab because they want to because if they don't their "friends or classmates" will tease them then. Are they not socially forced into hijab?
Listen, I don't really care if someone wears a hijab or not. Doesn't affect me what so ever. I will not ever accept that many, not all, but many wear hijab due to outside pressures.