Generally the API tests are good for just quick and dirty. The API calcium is probably the best of the lot And is roughly comparable to the higher end tests. Red Sea Pro works pretty well for me. Usually, I would give the nod to Red Sea Pro.
You have confirmed the test against a salt mix and the Red Sea salt seems pretty good (I have used it), I know the Instant Ocean and Reef Crystals can give wonky results for ALK. But I think the salt mix is a reasonable hint for which test is off.
You might be doing the Red Sea Pro test wrong. But I doubt it. You might want to watch a you tube video on doing the test but I don’t think that this is the problem.
The tests are not there to give absolute results. They are better at tracking changes in your parameters over time.