The 20% taxes withheld is treated like withholding taxes on your W-2 (advanced tax payment). It isn't your taxes owed-it is a IRS required minimum prepayment.
The distribution is added to your income along with other income items and taxed at your tax rate. This can often push you into another tax bracket.
See link below.
http://blog.turbotax.intuit.com/tax-tips/about-withholding-on-retirement-withdrawals-126/
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