2015 was a weird year for the Big Ten
MD and Rutgers were already included and Big Ten had already switched to the East-West Divisions, but still was playing only 8 conference games.
Iowa went 12-0 and 8-0 in the regular season without having to play Ohio St, Mich, PSU or MSU. But Iowa did beat ranked Wisconsin, which finished 10-3 and Iowa beat a ranked NW which also finished 10-3
MSU finished 11-1 and 7-1, beating Michigan because MSU returned a fumbled snap by the Michigan punter for a TD on the last play of the game. MSU then beat OSU on a last second FG. Then MSU beat Iowa in the CCC by 3 by scoring a TD with less than 2 minutes to play. MSU just had one of those years where everything went right in the games that mattered.
I think it's how Iowa loss the CCG to MSU is why it got the Rose Bowl. Losing by 3 in the last 2 minutes. If they had been blown out, probably OSU would have got the Rose. I don't think it was until after the bowls that it was obvious that OSU was a far better team.
Remember Iowa did not have the awful offense in 2015 that it does today. Iowa QB C. J. Beathard actually finished as 2nd team all Big Ten in 2015, and remember who Beathard beat out as Iowa's starting QB, Jake Rudock, who ended up starting for Michigan in 2015.