Let's call it here, Ohio State, Alabama, Georgia, Florida State, Oklahoma, Texas, LSU, Michigan, Penn State, USC (when not on probation) and Notre Dame have such an advantage in terms of resources and history that they only play two conference games a year where the odds are relatively even. If they play an OOC game against one of the very top tier teams, that makes it three games a year with even odds.
There are a few teams in each conference who can beat the best team in their conference, especially if at home; Wisconsin, Nebraska, Michigan State, Ol' Miss, Arkansas, maybe Florida, Tennessee, Washington, Oregon.
Every season seems to produce a Houston, Louisville, Baylor, Boise State, TCU, BYU... usually because a top tier team is on probation - Penn State, USC, or has a coaching problem - Michigan, Texas, USC, Notre Dame -
So maybe there are at most 25 teams to worry about each week. Injuries, internal personnel problems, distractions, bad weather, can produce an upset, but the same teams will consistently rise to the top ten spots and we know that before the first play of any season. So do the guys who put together the preseason polls.
Using my list, counting finishes in the top 15 from 1995 - 2015, Ohio State 15, LSU 14, FSU 13, Oklahoma 12, Michigan 11, Alabama 11, Texas 10, Penn State 9, Notre Dame 8,
I didn't realize that Georgia and Florida had been so successful in that 20 year span. As I was totaling up I realized they probably would have ended up ahead of Texas, possibly Alabama and Michigan. But again, they fall into the category of teams with the resources necessary.