+1 on what jaysee said. The tests won't differentiate between ammonia and ammonium. The presence of either one (or both) will give you a "positive" result.
What's the pH in your tank?
At lower pH's (more acidic, pH reading of less than 7), ammonium is favored.
At higher pH's (more alkaline/basic, pH reading higher than 7), ammonia is favored.
If your water isn't naturally on the acidic side, you can add a tannin source (IAL, oak leaves, driftwood, etc). This will help lower the pH a little, and cause some of the ammonia (bad) to convert to ammonium (OK).