Discussion of Net Mineral Acres

I still do not understand net mineral acres. I have had several different explanations and have read several articles. I do not remember net mineral acres being a part of old leases my family did and now it seems to be some sort of measuring device. Can someone please explain

Let’s say you purchase a 10 acre tract of land subject to a 1/2 mineral reservation by the grantor (for this example let’s just assume that they owned 100% of the minerals). You would now own 10 acres of land but only 5 net mineral acres.

If my grandfather owned 100% mineral rights on 160 acres (a quarter section) and he passed them to his four children, they would each own 40 net mineral acres. If each of these mineral owners passed their minerals on to their children, the 40 net mineral acres would be divided by the individuals children; if one person had two children, they’d each get 20 net mineral acres on the 160 acre tract. If one person had three children, they’d each get 13.333 net mineral acres, etc, etc.