I frequently find myself calling predictive ratings "anti-social" because of the way they seem to go out of their way to frustrate people. For college football, they are not intended to name a national champion or derive the quality of your résumé -- they're just intended to predict who would win the most games (and by how much, on average) if everyone played everyone countless times. You probably don't get credit for head-to-head wins, and in a lot of formulas you don't get extra credit for playing particularly well in specific games.
This is a long way of saying that LSU, 2019's national champion and a team that finished the season playing otherworldly ball, didn't finish the year No. 1 in SP+, just as it didn't finish No. 1 in FPI. Ohio State all but clinched the No. 1 spot by playing the best overall ball in the regular season and then playing Clemson to a near-statistical draw in the CFP semifinals.
Oh well. LSU timed its peak perfectly, playing well throughout the year and then going nuclear late. In the Tigers' last four games, their average per-game SP+ percentile rating was 99%. Play like that all year, and you're the greatest team of all time. As it stands, their midseason defensive funk (driven in part by injuries) dragged their full-season numbers down to that of a merely tremendous team.
Below are the full end-of-season SP+ ratings for your perusal. San Diego State (from 65th to 56th), Michigan State (48th to 42nd), Cal (74th to 69th), and FAU (45th to 40th) all saw decent bumps after solid bowl performances, while CMU (69th to 84th), Boston College (81st to 92nd), Temple (44th to 52nd), and WMU (82nd to 90th) all saw the opposite.
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. SP+ is intended to be predictive and forward-facing. That is important to remember. It is not a résumé ranking that gives credit for big wins or particularly brave scheduling -- no good predictive system is. It is simply a measure of the most sustainable and predictable aspects of football. If you're lucky or unimpressive in a win, your rating will probably fall. If you're strong and unlucky in a loss, it will probably rise.