why do some sororities not have all members recruit?

I'm a oos freshman pnm and I found out that the girls I know in some of the "top" houses (Tri Delt & KD) weren't "picked" to recruit. Someone said they pick the best 60-70 in-state girls to show during rounds to pnms. Is this true and if I pref one of those houses will this happen to me next year?