ISF calibration, or indeed accurate calibration is not about just doing the greyscale.
Our TV system is made up of a colour overlay on top of a black and white image. If the colouur your display shows black and white (grey) as is incorrect...and everyone I've ever measured out the box is....then when you add the colour to it you don't see what you were supposed to. If the colour overlay is not mixed correctly and hits the underlying black and white you get wrong colours and smeared image......if the steps of dark to bright are not in correct intensity changes for a given input signal then you get a cartoony or a washed out picture.
There are Standards written down for how our entire recording and playback chain should behave so that we see images on screen as they were intended. Good calibration ensures that your display adheres to these standards as closely as it can within the physical design constraints of the device. It is measureable as well as visible difference.
Hope this is some use.
Gordon