they actually played in. The national champion was determined by who was considered most deserving the year that team actually played in. In 1984 that was BYU. In 2008 or 2004 that was not considered to be Utah.
These things are/were in a vacuum with that vacuum being the year they happened. Obviously things change every single year in college football. If you want to make the argument that 2004 or 2008 Utah were better teams than 1984 BYU you can certainly do that but what can’t be argued is that in 1985 the voters felt BYU deserved the national championship in that year if competition and in both 2004 and 2008 they did not think Utah deserved he same. And in fact Utah had advantages BYU didn’t have in both Utah’s undefeated years, they got to play a big time opponent in a big time bowl game and still weren’t considered to be good enough.