The calculators can only really take a few things into consideration. Some power application efficiency must be assumed. If your car makes X amount of power, the calculator must assume you can get Y percent of it applied at Z rate. It would be an equally valid question to ask:
Why doesn't the calculator ask about your spring rates? Or type of tire? Or air pressure? Or what your shift points are?
Its really just a rude tool to be used for approximations. The same car will run much different times on bald street tires as opposed to hot slicks. The calculator doesn't know that...