I could give a lesson on bookmaking here, but I won't. Its enough for you to know that the thing that makes a favourite's price shorter and shorter is money being placed on it.
There has been no doubt for at least the last 3 series that serious money (I'm talking 10s of thousands at a time) have been wagered by people obviously in the know.
Whether that's people at the company collating the voting figures or staff at Endemol can be discussed, but I know which group I think it is...... the one's who can actually sway the outcome by editing, tasks, diary room footage etc.