Mechanic Question

I’m curious as to how you came up with the xdy-xdy mechanic. It’s a really elegant way of dealing with things like population growth, and I’m curious as to how it came about.

I first encountered the xdy-xdy mechanic in Robin Law’s Feng Shui and was very impressed with how it achieved a standard deviation around a mean such that you could use either the constant (ability score, skill, etc.) or the constant plus the roll, and have the same expected outcome.