Can an in take ammonia tester be innacurate as well as a liquid tester?

I bought a liquid ammonia tester and have been doing the proper water changes and i have 4 platies and an algae eater. The levels for ammonia have been in the death range(liquid tester). The fish are acting normal swimming throughout the tank(10 gal.). Today i bought an in tank tester and its reading normal levels. I don't get it?