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Most likely you made calculation errors and the IRS corrected them. While it is rare but still possible that teh IRS made a mistake - so do not spend the difference before you clarify all information. You will receive a letter with explanations in one-two weeks. You may call IRS support line and ask for explanations. 1-800-829-1040 - be prepared to be on hold for some time. If it is mor econvinuent for you - you may visit your local IRS office - no appointmnet needed - and clarify the reason of teh difference - http://www.irs.gov/localcontacts/article/0,,id=98284,00.html