Its completely manipulated, they're looking for a marketable artist. God forbid Michelle McManus happened again (sympathy vote, no career).
Certain shows, on certain years, have a clear winner. For X-Factor there's always the "favourite" but the public can and have changed that. I doubt Steve was the favourite for XF1, but Simon does like to win after all. XF2 Shayne was the clear favourite and rightly won (Brenda was the only other "good" contestant, god knows how or why she was voted out on her best performance). XF3 was Leona, again rightly so. XF4 was Hope (which didn't work for the public at all) or Rhydian but as people have already mentioned the public were wise to the obvious choice and went for a sympathy vote again with Leon.
Actually, I think being obviously pegged as the favourite can go against you. If the public think you're sure to win then they are less likely to vote. I don't, personally, believe the theory that Leon was gaining a fanbase. I think Rhydian was too much of a clear winner that he was losing votes because of the obvious outcome of the show. Leon snuck in there with a more rabid fanbase. Either way Simon has two artists he can sell. Bonus!
This year I think will be very interesting as it progresses. I can see about 5 acts, maybe one or two more, who could be classed, at this time, as a potential favourite. None of them are clear favourite yet though. I also think that Simon would be happy for any one of them to win but it will depend on how the voting goes over the initial weeks.
Alexandra and Austin have been invited back from previous years as "reliable" favourites. Laura, Rachel and Diana were happy accidents but certainly not "reliable" winners (all have distinctive voices that could work against them in this show).