They bracket the top 4 teams in s-curve order to their respective regions. So if Gonzaga is #1 overall, they get their choice - San Francisco. And then through 4. For 2 seeds they will try to get them to their preferred region but preference is given based on their position on the seed .list . However there are other consideriations like balancing the strength of the bracket. The committee won't put the #5 overall seed in the same region as #1. In that case, if Duke is the last #2 seed, they are likely going to be headed to San Fran considering the other teams on the 2 line. After the top 4 seed lines are placed on the bracket, then they assign 1st/2nd round sites. They go down the seed list for this too. So Gonzaga get's their first choice - Portland, and then they go down the list. Currently everyone above Duke except for Auburn have sites that are closer to them than Greenville. So when they get to Duke, they'd get Greenville. Hence, Greenville 1st/2nd round and San Fran regional. This is technically how it works, however the committee gives themselves wiggle room to make deviations from this which they do every year. Sometimes makes no sense.