The 1921 NCAA football season had no clear-cut champion, with the "Official NCAA Division I Football Records Book" listing California Golden Bears, Cornell Big Red, Iowa Hawkeyes, Lafayette Leopards, Washington & Jefferson Presidents, and Vanderbilt Commodores as champions. Only California, Cornell, and Lafayette claim national championships for the 1921 season.
The 1921 NCAA football season was the only one with no clear-cut champion.
We only know that the 1921 NCAA football season had no clear-cut champion; it does not state this was the only such season. The algorithm assumed it was, though.