I feel like the point of the prematch is for those students they feel they may lose to more competitive schools, in the hopes that the very competitive candidates rank them higher come prematch.
The numbers say that 65% of the offers made to students from UT-H are accepted; the rest are turned down (240/364 in 2014, check my link above).
Ostensibly, come match day, there would be a maximum of 240 matches and therefor offers. This suggests that, even at a high rate of 50% attrition of highly competitive candidates to other schools in which they retained their pre-matches post-match... and *then* subsequent filling off the wait list...thats at least ~60 prematch offers, not 20-40. I think what they may have been saying is that they retain 20-40 from the prematch; that would fit way better with the math. I'll ask on Friday and get back to you.
Either way, they offer about 124 more acceptances than they have spots, but most of that is happening during the match.