I use spring water, maybe makes the difference I dunno. I watched my neighbor use it to, on all of his plants he was told were calmag deficient, I watched him dose per instruction on the bottle and every plant fed it to suffered. Calcium toxicity is easy to achieve, google it many pics are found. I dont use, I have some, but dont need it. My soils are built correctly needing no additional additives, plus my spring water, directly drawn from under the the grow room likely compliments my nutritional feeding program.
easy to imagine those with little calcium in their water, poor soil builds, municipal or RO water do benefit from completing the feed with cal/mag. I didnt use it in hydro and dont use it in soil, got the best plants ever with nary a signs of deficiency.