I completely agree. In my opinion and from logical thinking testing the run off for PH is good but for salts its a false reading. I have had whole rooms of plants that the runoff was maxing out my pen when i collected the first bit of runoff. But when i did multiple slurry tests my pen was not even registering. The plants were hungry for food, i fed heavily and they all bounced right back in a couple days.
Doing runoff tests to try and determine my mediums salt levels has really slowed me down in the past. I would see those high run offs, flush heavily and watch the plants just get even worse and worse.
And to the original post question. Are you using a high grade coco mix?? As i am sure you already know if its a low grade non rinsed product it could be binding up your calcium??