why

Your experience is just one of thousands. Every year, girls go through rush and it's literally not possible for a majority of them to have full schedules every round. Like it's just not mathematically possible. It seems like you and your Rho Chi friends were an exception. Do a lot of girls get full schedules? Yeah, kind of. But MOST girls don't have a full schedules post IWTs. Also, I think it's important to understand that just because someone tells you they had a full schedule and got their first choice doesn't mean they're telling the truth. I knew a few girls who didn't have full schedules and/or didn't get their first choice but was embarrassed and lied about it.

However, I will say it is common for girls to get their first choice after Pref. IF you maximize your options, attend all parties, and don't suicide then there's a very high probability that you will get your first choice.