See where it says "Note: A piston with a positive number is either a flat-top with valve reliefs or a dish. A piston with a negative number is a pop-up."
The reason for this is because of what I stated the first time. A domed piston has a negative cc measurement since it takes away from the combustion area. A flat or dished piston has a positive number because it adds to the combustion area. It's just that simple.