Here’s my two cents.
I experienced the same problem with Salifert. The kit read 1300ppm, and the independent lab using ICP analysis read 1650ppm Mg.
1100ppm Mg is not too bad, and 1400ppm Mg is not too bad either. Mg has the largest safe range of concentration in the tank of any measured parameter. I have not heard about any of the kits reading high i.e. a kit reading 1400 when it was actually 1100, although I know one It is the interferences from each individual unique tank chemical solution that vary and it screws up the measurements, and these can vary over time.
One such interference is vinegar, while it has some good benefits it can also screw up some tests. The tests that I know that it causes problems with so far are the Red Sea Phosphate, All Alkalinity, and Seachem Magnesium.
So why not just shoot for 1200ppm Mg with the test kits. That’s what I do knowing that if it happens to be 1450ppm well that is probably ok. But if the test kit is reading 800ppm Mg then I should add more Mg to bring it back up to 1200ppm.
Now if there is a better kit out there then I would definitely buy it.