Could be debris settling to the bottom when the pump shuts off, or could be that the internal air bleed is clogged, or missing. When i used to take filters apart, i would see more dirt near the bottom.
I have a single Hayward cartridge (C1200) and mine will seem to have more sediment near the bottom when I clean it off. Not sure which make/model filter you have, but as Pool Clown stated, gravity tends to pull some debris down.
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