Because, as far as I know, the continent's average realm size is taken into consideration. I'm not sure if acceptable taxes are linear to realm size alone, but rather to the realm's size as opposed to the average.
And this is to ensure that acceptable taxes are not dramatically higher or lower on one continent or another simply because the continent supports larger or smaller realms in general. It means that
on average, acceptable taxes should be pretty comparable from one continent to another.
