Q. Is there some obvious bit of math I'm missing?
A. Yes*. You should not be getting an additional $601.
If your AOTC is only $1226 and you have enough expenses for the full $2500, the usual explanation would be the $1000 (refundable) is on line 29 of form 1040 and the $226 is included on line 20 because that's all it takes to get your tax to 0. If line 20 is something other than $226, you have other credits from Schedule 3. The non refundable AOTC amount is on line 3 of Schedule 3.
*For details, you may have to see the Credit Limit Worksheet for line 19 of form 8863 for the calculation of the non refundable part (and that's a complicated bit of math).
