Why is Bowles-Simpson more progressive than Romney's "very similar" plan?

Yesterday, the New York Times' Jackie Calmes provided a bracing model of how to reality-check a politician's ridiculous claim:

Mr. Romney said “my plan is very similar to the Simpson-Bowles plan.” The Romney proposal, however, has little in common with that bipartisan deficit-reduction proposal from a majority on the fiscal commission that Mr. Obama created in 2010. The Simpson-Bowles plan called for reduced income tax rates, but it would have raised about $2 trillion more in tax revenues over 10 years, mostly from high-income taxpayers, and cut spending to reduce the federal debt.

This discrepancy raises a more fundamental question.  Because Bowles-Simpson raises new revenue and Romney's plan doesn't, it should be easier, not harder, for Romney to make his cut-rates-and-reduce-loopholes plan more progressive than Bowles-Simpson. According to the Brookings/Urban Institute Tax Policy Center, the opposite is true. Why?