What is the "science" behind why water/nutrient solution pH in hydro needs to be lower than in soil?

I am growing in a DIY substrate which is (roughly)

3 parts perlite, 3 parts vermiculite and 3 parts coco coir.

What puzzles me when looking at these usual nutrient pH range charts for weed is the "flip" of the availability especially for Ca(lcium), which in hydro is like 5.2-5.8, and in soil 6.5-8.5.

What is the explanation here?

What pH range would be the best for my substrate, more like soil pH or hydro pH?

Thanks!

Look up cation exchange capacity. Your medium is considered soilless so you should be phing between 5.8-6.2

You may be right there, but the question is how the vermiculite plays in there. Because vermiculite (AFAIK) does store nutrients, unlike perlite or coco. So it is not an "inert" growing medium. When I look up Hempy buckets, which are often done with 50% perlite/vermiculite, their recommended pH would be 6.2ish. So common sense tells me that 6.2 might work, or likely even less since the vermiculite amount is less than 50%. Thinking about it, I think 6.0ph maybe, so your 5.8-6.2 does make sense.

Edit: I have already one grow under the belt with this mix but started off with very strange symptoms of a deficiency which I couldn't explain, but this when I had the pH too low, more like 5.8ish as for coco. Only after I flushed and then used far higher pH, like 6.2-6.5 things got better. This time I will look not going down that much.

In natural soil the ph is always changing at the root zone depending upon what organic matter is present in each tiny area as things are happening at the molecular level at different rates. Material is decomposing by microbial activity which is making the soil more acidic but maybe not over there where there are ph buffers inherently at work. PH is simply the percentage of hydrogen available for cation exchange which is how plants absorb these nutrients; a molecule of water is exchanged for a molecule of food by the roots.
In a sterile medium as you are running there is nothing organic to decompose or anything alive to decompose it so the percentage of hydrogen (PH) is whatever the water provided has. That is why checking runoff in soil tells you nothing useful yet in a hydro medium it is essential. You are growing in a hydroponic medium use the hydro ph range.

http://www.soilquality.org.au/factsheets/cation-exchange-capacity