because if you have a 12x1.25 lug vs 12x1.5 lug and each has 6 full turns, the amount of lug nut taken up on the 1.5mm lug is 1.5mm more than the 1.25mm lug, even though they are both equally secured since the threads are the only thing securing them. For road cars in general, the rule of thumb is a minimum of 6 full turns to be safely secured.