Could be many reasons. The IRS will mail you letter explaining. Did you have any amount on 1040 line 30 for the Recovery Rebate Credit? Then get the second Stimulus check after you filed? I’ve seen posts where the IRS reduced the refund like if you claimed the second $600/1200 but then got it after you filed. Or you didn't enter the right amounts you already got and tried to claim it again.
Or could it be for a state refund? Federal and state come at different times from different places.
