I've seen some tables online like the ones at American Homebrewers Association and Brewhaus Canada that list Potential ABV for different values of Original Gravity. My intuition tells me that these tables are developed using an estimated conversion rate for an average sugar content based on the Original Gravity.
It seems to me that most of these tables do not agree with each other. How are tables like these developed? Are they based on observations and experience, or is there a calculation that can be performed to determine Potential ABV using the Original Gravity (and a knowledge of the ingredients used)?
Disclaimer: I'm aware that Potential ABV is a poor estimate of Actual ABV. I'm mainly interested in the theory behind the concept.