Mineral percentages?

There is something we cannot reconcile. For 15 years Mom received only .00396% on her mineral rights instead of 18% Does anyone have an idea why the lower rate?

For a 3/16 lease, she would get 18.75% ONLY if she owned ALL the minerals in the entire unit that the well produced from. Her 18.75% is proportioned down to her share of the minerals. The 0.00396 would represent about 13.5 acres ownership in a 640 acre unit, or about 3.4 net mineral acres in a 160 acre unit, for example.

We are new to this so that really helps. Thank you.