Yeah, there's a HUGE difference . . . in price! I really don't know the "official" answer but have used Mg/Shultz for 3+ decades without issues. I think the "better" ones use more agreeable/less harsh resources as their ingredients? I've always added Epsom Salts @ 1/4 tsp per gallon to most waterings.
A more scientific grower will likely come along soon and detail this for ya.

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