From the average numbers produced by the 2 water facilities above of their respective nitrate residuals in tap water, both considerably below your test personal result, I would guess your numbers are in error.
Either your reagents are expired, or you aren't being careful, or aseptic enough.
One of the things I always did between tests in the lab where I worked, was to rinse each vial 3 times after with DI water, and 3 times before a new test, to make sure any residual was gone from the tube before starting another set of tests. This aseptic technique regime was required by the lab I was a chemist/microbiologist in.
I will admit that I am not that anal (especially here in Panama (DI is not really available here)), but I do rinse 3 times before and after each set of tests using rain water to rid the vial of anything that might skew results. (I collect rain water here, so its always available, and free of things like nitrate, unless a buzzard krapps on the roof above my rain barrel).
Most aquarium quality test kits are only accurate enough to give only ball park results anyway, so if expired could be totally erroneous, or if not fastidiously done could give far out results.
