The problem with hobbyist test kits - actually with all test equipment - is that they don't always give the right answer. Some times we don't use the kits right, and some times they just aren't that accurate.
Since all of your plants are growing well, assuming you have enough plants, there will be no measurable ammonia in the water. If you use Prime, or a similar dechlorinator when you add more than small amounts of tap water to the tank, you may have some ammonia from chloramine, if that is used by your water supplier, locked up but still being measured by the test kit.
If your plants are growing well and fast, you may have to dose more nitrates to keep the level from dropping and staying near zero. In fact if you have more than very low light you should probably be dosing all of the nutrients, NPK and trace elements.