retention rates

That's not what "retention rate' means. Retention rates are how many of the new members you take are still members after a certain length of time: 1 semester, 1 year, 2 years etc. For example, houses like Theta, Kappa and PiPhi have strong retention rates because they choose smart women who stay in school and are committed to their sorority and proud to be members. Houses like Zeta and Alpha Phi have weaker retention rates because they lose so many women because they flunk/drop out or have diminishing interest in the sisterhood.

Mistakes happen and there have been women who go through all of rush but don't wind up with a bid. But for the most part if you aren't too picky and make the decision that you want to be greek and are willing to go back to any and all houses that invite you back, you probably will get a bid. However, lots of girls drop out of rush when the houses they have their hearts set on don't invite them back, so lots of girls who start recruitment don't finish. Return rates, not retention rates, measure the number of girls a house asks back after each round vs. how many actually come back. Some houses need to invite almost everyone back just to keep their numbers up with the hopes of getting a full pledge class. Some houses cut too hard, which means they are too selective and have trouble keeping their numbers up. It's a dance that all the houses do and some are better at it than others. But the best houses (Theta, PiPhi,GPhi) generally put on a great rush and get who they want in the end.