It's unclear to me how this was calculated. But the Obama campaign estimates were based on some assumptions: 1) That Romney's ultimate budget would not increase the deficit 2) That the base tax rate would not be raised under Romney 3) Finding the necessary cuts to tax deductions* to satisfy the previous two conditions. (basically, estimating the tax changes necessary to satisfy all campaign promises made by Romney).

So it's unclear to me if the data being presented on this website is only comparing how the base rate would change without considering how the overall rate would change when including the various deductions.

Secondly, another reason for this difference, is that the Obama campaign usually talks about the middle class (married family with income ~$50,000) while the default settings for this website are for a single person making $100,000 (i.e. typical young professional in computer science).

*I don't know if this is the right word, but if you are married with children, or bought a home within a certain time frame you would normally get a small refund or deduction to your tax rate for having met these criteria. So cuts would be made to the various refunds/deductions, ultimately increasing your overall rate.

This one doesn't factor in the Romney's contradicting claim that his plan will be revenue-neutral. The Obama calculator does.