Overpayment or not?

I have been on SSDI for 25 yrs and recently had work audit going back to 2021. SSA calls and says I went over sga by $21 April 2022, $48 June and $22 July. When I was working a job helping students with accommodations at the college I still work. I would look at scheduled days for each month and multiply my hours x rate of pay and if I was going to go over because there were more working days that month my boss would let me work less days to stay under SGA. So I asked HR for my time sheets and I am under each of those months he said I went over. June we part timers got stipend $100 of left over covid funds the college received and didn’t know what to do with. It’s my understanding anything related to covid shouldn’t be counted as earned income for SGA. July I am under by $120! I just don’t understand as it’s money earned not money paid in any month. HR had pointed out April had 3 paychecks in April 2022 but I told her that April 1 paycheck is March money, that doesn’t count for a supposed April overage. Does anyone know what I am missing? I had to quit that job due to long covid. I returned to the college in a lower paying, less stressful position so I’m no where near going over now. I’ve always calculated my work this way to stay under and was given flexibility to do so by supervisors.