The concentration of calcium ions in the extracellular water and in blood is always kept approximately constant.

The bones are a reservoir of calcium in solid form, so whenever the concentration of calcium ions in the extracellular water drops the bones release calcium ions and whenever that concentration increases the bones retain calcium ions.

This takes care of short term fluctuations. In order to also keep the amount of calcium stored in the bones approximately constant, the gut will try to absorb more calcium from the food when it is insufficient and the kidneys will excrete the excess, when it is too much.

I asked my immediate family members to do a test as well. Male/female, young old, across states and even countries. All of them except my mother had low Vit D levels. My mother regularly supplements as its recommended for bone loss prevention in that age.