As with any TV show that depends on the vote of viewers to determine the outcome, it gets to be more a matter of who has the biggest following. Who watches the show and actually votes is what matters. The 'best' singer isn't always going to get the most votes. Even if you win Idol, your career will still be determined by having hit records and being popular in the long run.
Haven't watched much this year, but I know in some prior years the judges seemed to gush about certain contestants who I was never really that impressed with. Many Idol winners never went on to big success in the music business or had maybe 1-2 hit records and their careers were short-lived.