Ideally it is all mixed at once, it can settle so a portion of the bag gets one reading and another portion has essentially the opposite. You mention both halves mixed the same, this would leave only a few possibilities.
The test kits are wrong.
The salt clumped and reacted with moisture in the package and precipitated the Ca/Mg and Alk. There would be definite lumps and wetness to the salt in this case.
The salt was mixed into a very small amount of water and more water added later, the same precipitation could then occur. For example you pour half the bag into roughly a gallon of water and then slowly add more water from say an RO unit, or you added the bag of salt to a bin and slowly let RO water fill it from the RO Unit.
The salt should mix to about 410ppm Ca, 7.5 KH Alk and 1350 Mg.