That only works if you have pure lead. The beauty of the posted calculator is you can weigh your balls and then look up the various materials and get some idea what you are casting. Or how much impurity you have in your lead. Sometimes what you think is pure lead turns out to not be. 
You are correct, Michael. On the other hand the formula works well enough in sizes up to 1" even in lead alloys such as flashing, plumbing vents, plumbers lead, wheelweight metal and salvaged fishing weights that the error is so slight as to be of no concern.
I think that in this case perfect is the enemy of good enough.
Three Hawks