why did the irs reduce my refund?

All or part of your refund may have been used (offset) to pay off past-due federal tax, state income tax, state unemployment compensation debts, child support, spousal support, or other federal nontax debts, such as student loans. To find out if you may have an offset or if you have questions about an offset, contact the IRS to which you owe the debt.
IRS also may have changed your refund amount because they made changes to your tax return. This may include corrections to any incorrect Recovery Rebate Credit amount. You’ll get a notice explaining the changes. You can also check online at Where’s My Refund?
Tax Topic 203, Refund Offsets for Unpaid Child Support, Certain Federal and State Debts, and Unemployment Compensation Debts has more information about refund offsets.