I still think that the main reason that so many people get inconsistent parameters from their chosen salt mix manufacturer, is that they end user doesn't thoroughly mix the dry salt before using it.
Think about it, the manufacturer makes their salt, they test it and it's given a quality assurance check. From there, its dumped into the bucket / box, and sent along many conveyor belts before being stacked in pallets for distribution.... it then travels by road / rail/ sea to wholesalers... who then move it on to retailers... who then get it delivered (or they pick it up).
A salt mix is exactly that, a mix of different salts, Sodium Chloride, Calcium Chloride, Magnesium Chloride, Potassium Chloride etc etc... each of these salt crystals have different densities and as such with all the shaking about they get from the time its first put into its container, to the time the end user scoops some out, the heavier density crystals have fallen to the bottom, and the lighter ones displaced to the top.
Therefore the heavier ones, such as Calcium and Potassium Chlorides will be in greater concentrations lower in the bucket, Sodium and Magnesium nearer the top.
That's why when I get a new bucket of salt, I empty it out and give it a good mix by hand. I then weigh out portions to match the amount needed to do a X Litre water change at my preferred salinity, and place that amount into a brown paper bag. I then put all those bags sealed up back into the manufacturers bucket for safe keeping. Now over time if that bag gets jiggled about etc, it makes no difference, as the whole contents of that bag goes into my X litre of RODI water for mixing for the water change.
Doing this, I have always found that my salt mix has consistent measurable parameters (Alk, Ca, Mg).