That Michigan team started ranked #5... But they went into bowl season at 8-4, so wouldn't have been in the playoffs.
Same thing with Texas A&M this year - started out high, lost to a smaller school...but it turns out they weren't really very good to begin with.
On the other hand, Georgia and Michigan and TCU and even USC have proven by now that they are actually good teams. Which means upsets in the playoffs would be a lot less common