There are lots of reasons why the IRS adjusts refunds and sends taxpayers amounts different than what is on their return when they file. It could be that your refund was subject to an Offset:
If you owe money because of certain delinquent debts, the IRS or the Department of Treasury's Financial Management Service (FMS), which issues IRS tax refunds, can offset or reduce your federal tax refund or withhold the entire amount to satisfy the debt.
If you owe federal or state income taxes your refund will be offset to pay those taxes. If you had other debt such as child support or student loan debt that was submitted for offset, FMS will take as much of your refund as is needed to pay off the debt, and send it to the agency authorized to collect the debt. Any portion of your refund remaining after an offset will be refunded to you.
It could be because of a simple data entry error on your tax return that was corrected by the IRS. It could also be because you left something off of your return such as a Form 1099 or a W-2.
Whatever the reason, the IRS will send you a letter of explanation. You should receive the letter with a week or two.