no offence to OP but i am thinking this just a case of bad water maint,Check your tap water. If it shows 0 then your test is accurate. If it doesn't then either your test kit is bad or your tap water is a source of nitrates.
When I first started my current job my coworker showed me several of his planted tanks. He bragged about how seldom he changes water because "the plants will keep the nitrates close to 0". A quick test of all of his tanks yielded similar results to yours...
Do you rinse your test tubes soon after testing or do you tend to leave tests in them? Test tubes can become contaminated if they aren't washed promptly and properly. At work we have to acid wash our glassware a couple of times per year.