This might be the time of year in which computer ratings and the résumé ratings we have in our heads disagree the most. Top teams get upset, but the computer doesn't dump them as much as we expect. Unbeaten teams don't automatically move way up just for not losing.
With that in mind, let's go ahead and start with the disclaimer:
What is SP+? In a single sentence, it's a tempo- and opponent-adjusted measure of college football efficiency. I created the system at Football Outsiders in 2008, and as my experience with both college football and its stats has grown, I have made quite a few tweaks to the system.