The calculators take basic simplified physics formulas and perform calculations based on highly idealized assumptions. For example they will typically assume no aerodynamic resistance. They don't take into account the effects of things like proper weight transfer, CG, etc. They assume your car is a point mass that is propelled by a constant fixed power level. Hence why they predict a 300 HP engine will get you in the 12s. If your car gets close to a calculator then you can be sure you have a very efficient setup.