People dramatically overestimate how easy it is to go unbeaten (if you don't play in the ACC and are named Clemson).
Ohio State looks really fucking good, but they still play Wisconsin at least once, likely twice. Whisky is a very good football team. Even if you think Ohio State is 90% to beat Whisky, they lose one of the two games almost 1 time in 5. The combo of Sparty, Penn State, and @Michigan is also a trifecta of games that they can lose if the other team plays up and Ohio State plays down. If you think tOSU is 95% to beat Sparty, 95% to beat PSU, 95% to beat Michigan, 90% to beat Whisky, and 100% to win its other 3 games (NW, Md, Rut) (these numbers are all higher than reality, some much higher, BTW, just for demonstration), then tOSU still loses a game over 30% of the time.
FPI says tOSU is the #2 team with #1 overall efficiency, and also has them at 37% to win out.