I see. I notice it says "blood dilution." Meaning I guess, even if the blood tried to be cleaned up with a wet rag, or something like that. And the blood cells got diluted, but not all were picked up by the rag, some diluted blood cells were left.
But in my way, swipe right to left, one swipe, throw away. Swipe again, same direction, throw away. If any blood was left, it would only be at the end, where you picked up the paper towel, or whatever was used. Then you could just use a fresh paper towel going up to down one swipe at the end, and it would take care of that. Kind of like window washing, except you are doing it with fresh wiper and water each time.
I don't know. Just a theory, of course.