It's important not to lose sight of the fact that X Factor is a TV show first, and a talent contest second.
That's why the supermarket lady is through to the 'live' shows when she hasn't a hope of winning - it'e because a large part of the TV audience like her, but would not ring up and vote for her - so she stays in the show.
On the other hand, the judges will pick someone who has a better 'appeal', over someone who has a better voice - modern pop stars are built on image as much as vocal ability - remember Michelle Macmanus? Everyone was talking about how nice it was that someone less photogenic won a contest - but the reality is, people don't want overwight plain pop stars - it simply does not happen.