If you look at 1984, BYU’s title was not unanimous. Three other schools, Florida, Washington and somebody else, I believe, got first place votes from some of the groups that have a vote for the championship. The big ones went for BYU.
In 2004, USC got a unaminous vote. Three of the voting groups vacated the title, but did not award it to anybody else. Therefore USC got all of the remaining votes. Still Unaminous.