Oddity at TradeSports

According to the latest odds for political betting at TradeSports, the probability of the GOP retaining control of the Senate is 76.0%, the probability of the GOP retaining control of the House is 88.0%, the probability of the GOP retaining control of the White House is 52.2%, but the probability of of a Republican sweep (Republicans to retain control of Presidency, Senate & House of Representatives) is given as only 20.0%. If the first three probabilities were independent, the last should be 34.9%. Since they aren't independent, the probability of a sweep should be even greater.

Is somebody cooking the books?

Update (2004/07/11): The latest odds give a 33.0% probability for a Republican sweep.