unless your tank is 4 gallons there is no possible way a 2 gallon change will take 80 ppm and drop it to 40 ppm thats a 50% reduction. I would double check your tests.
+1 it sounds like your tests are bogus, or your doing them incorrectly.
Its a possibility, I bought a red sea kit while my tank was cycling and kept getting an ammonia reading, turns out I had no, and the test was just wrong.