i dont understand

Because they truly love and want to place way more girls than they're allowed to. They don't know who will rank them what, so as a sorority they want to be going after more girls than they'll ultimately get. If they only hardcore rush the 90 or so they'll bid they wouldn't make quota, and it's also be super rude to every other pnm. So they are kind and welcoming to everyone. By the end of the week they have to let go of girls they really want to keep. Trust me, there are fights every night over girls that people love but Panhellenic makes us cut a certain number. It sucks it happens to you but it's not fake. If a house did that they truly were interested and it just didn't work out.

Because you have alums “so called moms” who want their daughter and all their friends in the same sorority so they can date the right boy from the right high school whose family goes to the right country club to get a MRS. Degree.

I was in your shoes when I went through rush. I fell in love with a house. The day before pref, I talked to a girl I loved from the day before.I met their president and a few other officers. They asked me if I could see myself in that house. I was so sure that I would be there for pref, to the point where I may or may not have practiced their hand sign.
......And then they cut me the next day. I was devastated. How dare they do that and get my hopes up.
It happens. It sucks, but it happens. None of those things guarantee you a bid or necessarily indicate that you’re going to be there the next day. It’s a hard pill to swallow, but it’s true. I’m sorry it happened to you too, but it’s just how Rush goes sometimes.

They will promise or say anything knowing they are going to cut you. This is a one sided process. Be glad you didn’t end up in that house because they would expect you to treat another sweet girl the way you were treated.

Not every sorority has rush practice like this. Even of the top 4, there is one that leads on many more girls than 90. They lead on 130 or so to make sure they have two good lists. They have off campus parties and serve alcohol to underage girls. They do it and they know it. They never get caught! Why? Daddy has deep pockets. My daddy does too but we don’t throw our money around!

Also, the girls who did that might have really wanted you as THEIR #1. But they have to fight against a tide of other sisters that night in bid session who also want their #1. Those sisters might have TRULY wanted you but just the wants of those handful of sisters don't dictate what the whole chapter votes to do.