Because the purpose of weighing a vehicle is to determine "race weight" not "race weight after removing weight".
To determine race weight you would have to fill the radiator back up and of course know how full it was in the first place in order to be accurate.
Geez, if you need a written rule on how to interprut that, you must have a sorry program.
I only raced Dave's 'Cuda once. Darrin beat me....
