The best teams in college football and the ones most deserving of a national title shot don't always overlap perfectly in a Venn diagram. If you go 13-0 with a bunch of close and unlikely wins, you might not be one of the best teams in the country, but you deserve a title chance. Consequently, if you beat 10 mostly good teams by mostly huge margins but slip up twice and don't win your conference title, you probably don't deserve a spot in a four-team field even if you're definitively awesome.
In 2019, the teams ranked first, third, fourth and sixth in the end-of-regular-season SP+ ratings reached the College Football Playoff. In 2020, it was the top three teams and No. 9. But in 2021, we came closer to perfect overlap. The four teams that will play in this year's CFP currently rank first, third, fourth and fifth. No. 2 Ohio State is an outlier of the "look great 10 times but lose twice" category listed above, and the Buckeyes will go to the Rose Bowl instead of the CFP because of it, but four absolutely fantastic teams will play in the semis.
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.
It is, as always, important to note that SP+ is intended to be predictive and forward-facing. 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. It is a look at how teams should likely perform moving forward.