First, normally they would all be the same.
When did you file your 2020 taxes? e-file or paper file?
Was the younger kid on your 2020 taxes?
$550 would be the full amount for your two kids if your joint income was under $150K. So maybe at one point they were adjusting something and didn’t look at your income (which I assume that for two people, one of whom has been an actuary for a while, that it would be > $150K).
If you’re over the first phaseout but under the second threshold (I think $400K), the amounts should all be $333.33, which is fairly close to the August amount. Then there’s another phaseout that happens. So if you were just slightly over that then $330.00 is plausibly correct.
Maybe some of the higher amounts are attempting to make up for not sending a payment in July. And if they processed (not received) your 2020 return in there then they would have.
It averages to fairly close to $300, so here’s my best guess.
July: they screwed up by not sending anything.
August: based on your 2019 income
September: they decided to give you 1/4 of the July payment over each of the remaining months
October: updated to be based on your 2020 income but the clerk screwed up and dropped a digit of your income and thought you made ~$40k, not ~$400k
November: realized the screw-up and trued up.
My guess is that your December payment will be roughly $294, which is simply the average of July - November.
That’s really just a guess though.