This was a surprise to me too. It was actually explained a few times (or many times?) in Edgar Cayce readings. It is also well known that our body cannot use minerals in their natural, elementary, state. They'd have to be "chelated". But even then, for some reason, calcium is very hard to absorb, and is a burden to liver.
This all comes from Edgar Cayce readings. If you don't believe what he said, then it may sound like garbage. But keep in mind he got it right some 10,000 times, so it's highly likely he got it right with calcium too.
I will first try this on myself, and will let you all know.