[KDCat]

More general information to go with what DubaiSis said:

Each sorority can only hand out so many bids. That's called "quota." They have to choose among the available pool of PNMs to fill quota. Choices are made based on good conversations and personality, but they are also made on extracurriculars and grades and leadership potential and general fit with the group.

The process that DubaiSis described of cutting after each round of parties is to give each sorority the best chance of filling quota and each PNM the best chance of receiving a bid. Those cuts are really tough, but they are designed to maximize options as much as possible, so that no one goes bidless and every group gets a full NM class.

As far as why any individual person gets cut, nobody knows but the people in the membership selection meeting and they're not telling.