There are not many people voting for any of them. Dermot said thankyou to 1 million people who voted but what he meant was that 4 acts combined totalled 1 million votes, not 1 million voters. A lot of people who do vote, vote 10 times, so the number of people voting is probably under 1/2 million. Split it equally and you have about 125,000 voters for each act, and assume it isn't equal then it's just tens of thousands for some acts, probably including Chris's.
I would guess his supporters (and the show's detractors who want to duff up the result by voting for Chris) are very active on the phonelines, partly because the judges made it far too clear they didn't want him to win. So he gets a sympathy vote plus the vote for the worst brigade.