Last year, if it wasn't Ohio State left out it would have been Oklahoma. Some argued UGA should have been fourth (not me) and they were close (fifth).
OSU was sixth.
There will always be 1-2-3 teams "left out" no matter what, but when ND is 12-0 that goes up one.
I have a sneaking suspicion that the committee sometimes organizes their final rankings so as to minimize controversy. In 2018 the top-3 were abundantly obvious to anyone:
- 13-0 SEC Champion Bama
- 13-0 ACC Champion Clemson
- 12-0 Notre Dame
After that it got hairy. Based on their final rankings of #4 OU, #5 UGA, #6 tOSU is looks like it was between OU and UGA but I don't think that was actually the case. I believe that if OU had lost the B12CG to Texas, Ohio State would have gotten in. I just don't believe that two-loss non-champion Georgia would have been in ahead of any one-loss P5 Champion.
FWIW, I think they did a similar thing in 2016 when 1-loss non-Champion Ohio State got in over 2-loss B1G Champion Penn State despite losing H2H to the Nittany Lions. In the final rankings they had one-loss Pac Champion Washington in between the #3 Buckeyes and the #5 Nittany Lions.
The committee seems to seriously frown on really bad losses. In 2018 Ohio State had a blowout loss to a mediocre Purdue team. In 2016 Penn State had a close loss to a bad Pitt team and a blowout loss to Michigan.
I think that the committee's view of bad losses has perhaps evolved over the years. In the first couple years of the playoff some teams got in with pretty bad losses. Lately that hasn't been the case. Of course the other thing at play is that it isn't a set criteria. One year you might need to be undefeated. Eventually there will be a year when a two or even three loss team gets in. That all depends on how strong the competition is.
Last year was a highly unusually strong year. The P5 plus ND produced three undefeated teams, two one-loss Champions, and a two-loss Georgia team whose only losses were by a TD to the consensus #1 and on the road to LSU. Additionally, Michigan finished with two losses both on the road to teams that ended up #3 and #6. That is five really strong contenders and two others not far behind.