This is because H2S04 is a diprotic acid. If we look at how the acid dissiociates, we can see that:
H2SO4----------> 2H++SO42-
Since there are two H+ ions dissociating, using the formula for pH would give the H+ ion content within the whole acid, i.e 2H+
Dividing it by two would equate to a singular H+, which is what the question asks.