If the dirt was in the water coming out of the jets, it seems highly unlikely that it would fall straight down and not get thrust out with the water. Maybe there's a dead spot in the water flow there and the dirt makes its way around and stops there.

I have found with pollen especially that it gets stirred up very easily so whether I vac by hand or use the aquabug, some of it gets sucked up and some of it gets stirred up into the water so no matter how clean it looks when I'm done, there always seems to be some that settles back a short while after. Hopefully the pollen season will be over soon. Right now I would just be happy with a full day of sunshine!

Peter