Oregon and Washington are well ahead of the rest of the pack from their top finishes, even if their average isn't quite as separated from the pack. Going back to 1968 (when AP began consistently ranking more than just the top 10), 20 schools have 5 or more top 5 finishes. Nobody else more than 3 (and only Pitt, ASU and UCLA even have 3). Oregon has 6, same as LSU, Auburn and Tennessee. Washington and the much degraded Colorado program have 5. You, of course, mentioned all the ones with more than 6 (the 8 blue bloods, PSU, FSU, Miami, Florida, Clemson, UGA).
All of these schools but Tennessee have 3 or more top 3 finishes. Tennessee has 2. Outside those 20, only Pitt and TCU have 2. In fact, there are only 14 top 3 finishes in 54 years that are not one of those 20 schools. Those 20 have 52 of the 54 AP MNCs in that period (Pitt and BYU the outliers). Now Oregon and UW don't have AP titles, but UW has a coaches title. Those 20 (but excluding Colorado now) are the only schools who are a serious threat to finish #1 in any given year.
Minnesota is not 3rd tier anymore. They were a blue blood at one point in time, but 1967 was their last really good year. Except for Pitt, the rest of your 3rd tier has no more than 2 top 5 finishes since 1968 and combined only 6 top 3 finishes. Just not on the same level as UW and Oregon.