Well, I'm trying to get into these peoples' heads, and I can think a few approaches they may take...
1. We really like this kid (based on WE, soft factors, accomplishments), so based on the class we have so far, will he/she hurt our 25/75/medians? Nope. Great. Let's admit him/her.
2. We want the highest LSAT/GPA median possible. Who out of the waitlist pool will bump up one without bringing down the other?
3. We have too many white males.
Obviously, the first scenario would give me the most hope and entice me to submit more info, LOCI's, etc. In reality though, I'm guessing they go with a combination of 2 and 3.