The use of the net rate captures the generally more favourable tax treatment of pensioners than of workers.

ECONOMIST: Ageing and public finances

The theory being, if you cut enough deductions, the same level of net tax revenue can be maintained despite a nearly 20% drop in the top rate.

FORBES: Tax Aspects Of Paul Ryan's FY 2014 Republican Budget Proposal

It begins with a 15% flat rate, but taxpayers who cross certain income thresholds owe more because of the 3.8% net investment income tax, the Pease limit and the Personal Exemption Phaseout, or PEP, a backdoor increase that limits personal exemptions.

WSJ: The New Capital-Gains Maze