Derek,
Maybe I can answer some of the questions that the rep wouldn't? For starters, I also have worked with the Sera reps. There are good ones and bad ones, just as in any other industry. Some do it for the money, some do it because they enjoy it, and some actually know about the products they're selling.
When they make the test kits and the solutions are created and packaged, there are variations involved, which, being chemistry, can have an effect on results. There are always chances that something differs somewhere, and that is with any liquid, dry tab, or strip test. Of the most accurate on the market are various liquid tests. The factors that vary are more "exact" in a liquid kit than in a strip test or dry tab test. Our store worked for months with Sera to get through a bad batch of Calcium tests at one point. It was a mess, but they were helpful and replaced everything we found to be bad. That particular problem was due to temperature fluctuations when chemicals were mixed & packaged.
You'll find the same issues with hydrometers in saltwater... temperature differences when they are calibrated at the manufacturer compared to that of temp fluctuations during packaging, shipping, and even when on the shelf of a store. Here, the solution is to use a refractometer. Our store was using a refractometer to calibrate new hydrometers right out of the package as we were selling them, and some were found to be WAY off.
As I stated earlier in this thread, electronic meters are going to be the way to go if extreme accuracy is needed, which, in your case, it sounds as if this is important. For the average aquarium keeper, the slight variations in the many brands of liquid tests can be dealt with unless they are working with something sensitive, but many people cannot afford the electronic meters and/or don't need such exact results (or don't care enough about accuracy). I have seen so many situations, and what is good for one doesn't mean its good for all.
Does this help at all?