confused about pref

IDK what you mean that someone else may get your spot. Doing SIP doesn't affect your place on the bid list at all. You either match with them before they reach quota or not like every pnm. No one else gets your spot for doing SIP.

If you SIP and are right below quota, you are out. But if someone lower has two houses, they will be added on and get a bid.

That's a quota addition, not taking someone's place. SIP is a valid option when you are sure a house is not a fit (based on gut feeling, not on stupid ranks.)

why would they take someone lower on the list with two houses over someone who did SIP if it gets to quota additions? doesn't that go against what the sorority wanted?

They DO NOT take someone lower on the list for having two houses. That is incorrect and not how the algorithm works. The sororities do not know or care if pnm SIP's, they simply rank their pnm's. The algorithm does the matching. Here's how it goes:

After pref and everyone signs her card, quota is set. Pnm's rank their houses 1 & 2, or just list their 1 if they SIP, and the sororities have ranked all their pnm's in order on their bid list. The algorithm starts at the top of a sorority's ranked list and tries to match with the pnm's who put them at 1. The bid matching process for a house goes until they reach quota and then they are done. If you are high enough on your 1's bid list to match before they reach quota you will get a bid from them. If not and you SIP'd, you are out of the process and will not get a bid at all.

If you have ranked both your houses and are not high enough on your number 1's bid list to match with them before they reach quota, the algorithm will then try to match you with your 2. If you are high enough on their list to match with them before they reach quota you will get a bid from them. If you aren't high enough to match there either, you will be placed as a quota addition to either house. The algorithm will try to place you with your 1, but will also consider your place on both bid lists and the number of quota additions to place. This is how you are guaranteed a bid if you rank both houses. But no quota additions for SIP, that's one and done with no guarantee of a bid.

If you only have one house at pref and put them on your card you will get that bid.

Also remember that there are no releases after pref, there is only bid matching. If you are invited to pref you are on the bid list somewhere. When you sign your card you are committing one full year to either of the sororities you list there. If you would absolutely never in a million years no matter what accept a bid to your 2nd choice pref house then don't list them and there will be no binding agreement. But don't think you are going to get a 'better' house with a COB, you're not. If you don't get a bid you need to check out Theta because that's going to be your option.

"If you have ranked both your houses and are not high enough on your number 1's bid list to match with them before they reach quota, the algorithm will then try to match you with your 2. If you are high enough on their list to match with them before they reach quota you will get a bid from them. If you aren't high enough to match there either, you will be placed as a quota addition to either house. The algorithm will try to place you with your 1, but will also consider your place on both bid lists and the number of quota additions to place. This is how you are guaranteed a bid if you rank both houses. But no quota additions for SIP, that's one and done with no guarantee of a bid."

This right here is exactly how someone ranked lower on a bid list can get in above you if you SIP. At the end of the day, it doesn't matter what words are used. A bid is a bid. If you are invited back to 2 houses, you better hope that you like them both a lot because you shouldn't feel pressured to agree to rank 2 when it's not how you feel.