I tried this the other day too, out of curiosity.
It has certain assumptions, such as family size/deductions as well as how the income is earned and the proportions earned in a married household.
So yea, it's basically worthless. Not that I trust Romney to be much better. It's the giant douche / turd sandwich conundrum.
When comparing taxes you have to use the finial tax due, line 61, on the 1040 form. Otherwise you are comparing apples and oranges