Tax question...

[IdealFugacity]

The first dollar we make and the last dollar we make are taxed at different (and increasing) rates, as long as we are lucky enough to make more than $7,825 a year.

When analyzing money market funds, bond funds, and other funds which are either tax-exempt or -managed, why do we use a simple calculation that uses our "Tax bracket", instead of our "averaged tax bracket"?

It seems pretty significant that if you're making say $60,000 a year, your tax rate will actually be: 17.7% by my calculations, using http://www.moneychimp.com/features/tax_brackets.htm, arriving at about $10,600 in taxes.

Am I doing this the wrong way?