1 thing could be how much of your container you are using per mix and how the minerals etc in the container have settled/dispersed after production/during shipping ...
One thing I've learned about ALL salt mixes is that most need to be thoroughly mixed if you're not going to use the whole container in one go. During shipping with vibration and such, heavier stuff can/will settle lower in the bucket/bag. This I've found is evident when the top of a bucket is very powdery.
If you are having inconsistency issues with whatever you use and are not planning on using the whole container in 1 go, roll the bucket for 5 mins to mix it well - do whatever you need to mix well that bag of salt - before you open it.
I've been using IO Regular for over 7 years and, ya, I've had a couple batches that were well over 1600 Mg, but it's not very common. Over 40 buckets used so far and mostly they are within a reasonable range.