Wait for the IRS explanation of the difference that will come in the mail in the next few weeks however while you wait you can review your return for a possible entry error. Some interest would have been added for the delay in processing however $800 is probably too much for just interest.
Did you have an amount on line 30 of the 1040 ? If so did you enter the correct amount of advance stimulus payments you got ?
Or in your situation it is possible you owed less for the old 2019 balance due than you thought ... check your online account with the IRS to see where your refund went ... https://www.irs.gov/payments/view-your-tax-account
