It's simple. TSA doesn't want its staff to have to think or apply judgment as that takes time. You have set up an extreme case, but imagine the following - someone is carrying a 300 ml bottle which is approximately 1/3 full. Does it go through or get pitched? Argument ensues. "You let my wife through with this much just ahead of me." Much easier to simply rely on the numbers on the package which will guarantee that the contents aren't more than 100 ml.

This isn't to defend the liquids ban, but given that they have one, I can see that the efficiency aspect of relying on manufacturer's label rather than having to judge the amount of liquid based on how much is left. Sometimes, it makes more sense to have a clear rule that is easily interpreted by both staff and passenger than to have tons of arguments as to whether a specific judgment is correct, even if in any particular case, the result is more restrictive than it needs to be.