Ammonia Test question

[intarsiabox]

I just added water to my new tank two weeks ago and although I've never actually tested for ammonia before (just waited a month in the past before adding livestock) I was curious to see what happens. So I bought an API kit and have been testing for a week now. I get the same result every day, 0.50ppm. It shows 0ppm after the first minute but if I wait the 5 minutes as directed I get 0.50ppm. I thought this was odd so I tested some plain tap water and got the same results, 0ppm after 1 minute and 0.50ppm after 5 minutes. Is my test kit faulty? Nitrates test at 0 right now and I've been adding Seachem Stability for a week now as well. Still going to wait awhile yet before adding any livestock but just curious about the results.