how do they really choose

Well if you’re a top tier like alpha phi (check reddit) or pi phi, then they’re judging you based on your looks.
My sorority decides only based on our conversations with you and how much of a connection there was

Everyone has private criteria based on their rankings. It is number’s based on invited so you’ll pretty much never know how you either got invited back to or cut from each house. Good luck!

Membership selection is so secret because for many chapters it is considered to be part of their ritual.

In general, selection is based upon a chapter’s values, but how they quantify those values will vary chapter to chapter within a given sorority as well. For example, a sorority may value leadership and ask you questions about how you’ve been involved on campus, in high school, outside of school, etc. to see how much you care about leadership. You may think the chapter wants you to say you only want to join greek life and be super involved in the chapter, but XYZ might require girls to be involved in at least 1 outside organization, so they wouldn’t score you high for that category. This is why it’s important to know you cannot game the system and all you can do is be yourself.

Many actives also will not know how PNMs are ranked overall during this process. They will not know your GPA, your other conversations, or what your score is compared to other PNMs. In many chapters only the VP of recruitment, President, and advisor will know this. So if even 99% of the chapter doesn’t know how you ranked, you’re not going to know either.

You probably had good conversations at every chapter. Many chapters are just great at recruitment and know how to make every PNM feel at home. Go into each round with an open heart and take the time to get to know each girl wherever you end up, because for many PNMs they don’t find they click until after recruitment is over.

Each sorority has its own way of doing things. Like yes, some do pre-score you on looks or your social media. My chapter uses an app where we rate each PNM on a scale of 1-5 based on our conversation. You're supposed to relate the PNM to chapter values but it pretty much ends up being if you liked the person and if they would fit in the chapter. depends who you talk to really