Dirt left in the tub at end of the cycle
We have a 6-7 year old WP DD TL washer at work that's used to wash a variety of mostly micro-fiber shop towels. Some are used to clean marks off windshields and as these cars are usually outside 1 to 3 weeks at a time, the windshields get rather dirty.
When these towels get washed they are SO dirty, I'll have to run 2 complete cycles to get them clean. Even after the 2nd complete cycle, there is often a fair amount of silty-dirt remaining at the bottom curve of the tub between the drain holes.
So my point is, while it's not 2 cups of sand, maybe a 1/3 to 1/2 cup(???) of fine - silt doesn't get "washed away" either, even in a fairly new washer.