DPS(a) stands for the optimal DPS you could have reached given the numbers in the parse if you had been active dps’ing all the time. It’s not an indicator that excludes overkill or anything like that as far as I know.
The report shows you’ve been active dps’ing 52.79% of the parse’s length.
1,290.3 (DPS[a]) * 52.79 /100 (% active) => 681.14; aka your actual dps: DPS(e)
See also: Ast's DPS(e) is being calculated incorrectly in A6s