For what it's worth - as I've been testing my water with API test kits I have been taking water samples to my local LFS
regularly as well. One thing I have noticed is that the kits we use at home only have a half dozen or so color references whereas the LFS
has an API wall chart with dozens of color references for each test.
So when my test was showing 0ppm on Ammonia their chart actually showed .25ppm. I guess the LFS
probably get those charts sent to them for selling the API kits.
I mention this as I think your idea of having your LFS
double check your readings is wise.