Across The Field
Oaky Afterbirth
OSU played in the same conference and won multiple titles in that era.So did all those other teams in question, which combined with the fact they had an easier schedule they could go undefeated easier than Michigan. Which was more or less a requirement to win a title in those days.
Also, Nebraska and Oklahoma played a fierce conference slate as well and both claimed multiple titles.
As it's already been said, michigan was great back in the dark ages of college football and it has led them to become overrated compared to the elite teams in the nation.