IRS: Notice CP25

So I just received a notice from the IRS that says:

"We adjusted your estimated tax total"

Summary: Tax Owed: $19,963.00 Tax Paid: -20,663.00 Amount Due: 0.00

What the heck does this mean? I read the explanation on the IRS website, but I still don't get it. I paid more than I owed? Then why is amount due 0?