all pnm's please read this!

Okay, some of what Truth said is accurate, some not so much...
Yes, almost every school in this country uses RFM to decide how many women each chapter can invite to the next round.
Each chapter, based on its last few years' return rates (what % of PNMs they invited to the next round accepted that invitation to the next round), will have a number given to them by PHA. That is the max number of PNMs the chapter can invite back, and as each round continues, the number gets whittled down to the quota range.
The reason for this is that you can pretty well predict how many women, say, Epsilon Sorority, needs to invite to Round Two to get a full quota come Bid Day. Yes, that's right, a few rounds ahead of time, the computer/algorithm/RFM KNOWS ALL. Things really don't change that drastically from year to year. So, Epsilon Sorority might like most of the 700 women who came to Round One, but they can only invite 300 to Round Two. There is no reason for them to invite more women to Round Two, because those extra women would just get cut before Preference, anyway.
Now, Iota Sorority might need to invite more women to each round (their return rates are not as high, so they need more women to pull from to get quota on Bid Day), so they might get to invite 450/700 to Round Two.
This helps the "weaker" Iota Sorority, but also DOES NOT HURT the "stronger"-recruiting, Epsilon Sorority, because Epsilon can just focus on the girls they really REALLY love, earlier in the process.

As far as I can tell, SDT participating like they do in formal recruitment does throw a bit of a proverbial wrench in the process, though. If they really tried for parity with the rest of PHA, tried to be at Chapter Total, etc., I think that would make a difference.
I am not part of SDT. I do not know if they put their all into recruiting all the way to quota, if they care that they are smaller and house-less, etc., but the impression I get is that they are happy the way they are. That is great for them, but it likely skews our numbers for overall RFM (because I imagine the formula assumes that they should have the same quota as all the other chapters, when really, SDT seems to work with a much lower number for quota. If they only want to give out, say, 35 bids on Bid Day, they should invite way fewer women to Round Two and Preference than PHA tells them, for instance).
Do they want to be small, so that's why they seem to operate the way they do, or have they always been small, so they have learned to be happy with what they have?
SDT was kind of a tangent from Truth's post, and I'm speculating and raising more questions here, and their membership is none of my business, EXCEPT that their participating/sort of not participating in Formal Recruitment RFM and quota must mess with recruitment numbers somewhat. I don't see how it couldn't.