I ran dissolved nutrients in a seawater chemistry lab for 7+ years. Probably ran 10,000 samples. NH4, NO2, NO3, PO4, SiO2. Ammonia is a tricky one to measure. It takes a long time for the reagents to react completely - took about 15 minutes on the very old (analog) instrument I used while NO2 took about 90 seconds. Also was very finicky at low levels, although my idea of "low levels" is probably a lot lower than most aquarists would care about (parts per billion - way below detection limits of any liquid test kit). So I'm not super surprised that ammonia might read off at low levels. Try waiting a bit longer before reading your results. Follow the directions on the bottle to the letter - if it says to shake for X minutes, don't short change it. Also make sure there are no sources of volatile ammonia around the house - did anyone just clean the bathroom or kitchen? Sounds funny but I had to rerun about 20 samples once because I dripped one drop of ammonium hydroxide outside the hood in the lab. Samples picked it up from the air and went offscale within minutes.
We would use test strips for pH occasionally and high quality ones can be pretty accurate for broad brush ideas of what the pH is but if we wanted accuracy the pH meter came out!