Because its only during the opening of the injector that flow is effected by voltage, once the injector is fully open and just held there, the flow doesnt change.

So if the opening speed is reduced by lower voltage all we need to do is compensate for that, not change the value during the "held" state.

It adds at all voltages so that it can be capable of not overfuelling in a high voltage situation, if it flatlined after 13 volts or whatever, then it would go rich at 16v.