Retirement tax questions

Curios why this is a taxable event if your funds in an Traditional IRA are after-tax funds, then isn't just your earnings taxable.  Say I rolled over $2600 from a traditional IRA to a Roth IRA and $2500 of it was after tax dollars and $100 was tax-free earnings then isn't just the $100 added to your income? I shouldn't get taxed twice.