The A/C should be maintaining the inside temp at the set point, unless
it's set so low that it can't reach it, so how are you measuring that
turning on the fan causes the temp in the whole house to drop 2
degrees? A more typical scenario would be to notice that with the
attic fan on, the house cools down quicker, the A/C runs less, or a
particularly hot area of the house, ie, an upstairs bedroom is cooler.
That's the case in my house, where the attic fan helps keep the
upstairs bedrooms cooler, but not the whole house.

As others have pointed out, it works by reducing the temps in the attic
from say 140 degrees to a more manageable 110. The amount of heat
entering the house from the attic is proportional to the temp
difference. So by going from 140 to 110, the fan can cut the temp
difference from 140-70=70 to 110-70=40, that's about a 40% decrease.