If the test strips are within their expiration period and if they haven't been dampened, they should be fine. You can always take a sample to the LFS and ask them to test it, to compare.
Even if your reading is 10 ppm too high, you're still dealing with lots of nitrates. Those of us in rural agricultural areas often see high nitrates in the Spring when farm fields are fertilized and you get runoff from Spring rains.
We often have 20 ppm nitrates in our tap water; I don't worry so much about nitrates per se, because I use nitrates more as a general marker of pollutants in my tanks. If you're really worried, you could get an RO unit...
I'd double check the nitrate level of the tap by testing with an alternative method before I'd lose any sleep over it.
Jim