Well, say I sold 10,000 copies of my single.
Anything that wanted to move higher in the iTunes top 10 would have to sell >10,000 or more. So if I'm in the chart for 2 years before you released your song you'd have a lot of catching up.
To counter this (and I know for a fact this is used in the Apple app store) they have algorithms that calculate the popularity, to give the new releases some exposure. Example, I may have sold 100x your app, but if your app sells at a faster rate then you'd pop in to the top 10; this is evidenced when you see apps appear in the charts if the price drops etc.
The obvious possibility is that it simply passed the X-Factor single in sales, but as I said.. I don't know 100% how they calculate the top 10 so it may well be clear cut as skippi90 said.