The calculator is wrong. I don't know if they are actively misleading or merely naive.
It doesn't take into account the fact that McCain is going to tax your health insurance premiums or that Obama is going to lift the social security cap (as I said earlier, these guys' tax increases come in sneaky ways).
My back of the envelope estimate is that a married couple with 2 children will pay more taxes under McCain if they make less than $250,000. If the couple makes less than $200,000, Obama gives them a better tax cut. If they make between $200,000 and $250,000 it's a wash.