When I clean the filter on our washing machine, I invariably need to cope with a litre or so of water that comes out before I can get to the filter. Why doesn't the machine pretty much pump itself completely dry?
No, it's not Phucker asking silly questions, just for attention. This is a real nuisance for users, and it probably explains why some people rarely clean the filters.
I haven't studied the designs, but I guess it's a centrifugal pump, and I don't really see why it leaves the sump half full of water?