Yes. That's exactly how I explained it above. You should have entered you got 4200, but you must have only entered half 2100 from one letter. On your 2021 return you only qualify for 2800 (2x1400). But you entered you got 2100 (from 1 letter). So you tried to claim the 700 difference on line 30.
But you really got 4200 which is more. Line 30 should have been blank. And you don't have to return the 1400 you got for your dependent.
