Hobbiest test kit are what they are and the results are not to be taken for Lab Grade results, The best indicator of proper levels is the condition of your coral along with testing with a test you feel gives you a guideline to go by. IMO Hanna has the most accurate in Po4 KH and Salifert/Redsea for Ca and Potassium. I have pretty much given up testing for Mg as long as you do water changes with a quality salt you will be in the proper range just like most other trace elements.
I must say the Charts look impressive but they are really useless in any fact based comparisons of brands.
Your tank will tell you when something is not right by the way your coral look and grow, And from there testing can be useful to find what's out of balance. JMO
Bill