Chances are you either did not get both letters saying you got $2100 advance stimulus each ( for a total of $4200) so you only entered $2100 and not the full $4200 you did get. Or you did get both notices and thought they were a duplicate notice.
The $4200 advance would have been for 3 people listed on the 2020 return. And it is also possible your dependent also got a $1400 advance if they get SS benefits. So not only did you get all of the stimulus in advance it seems like you got more than you should have but the way the rules are written you get to keep the excess since it was an IRS error.
