One big problem is the fact that R-value can be measured in different parts of the door which will make major differences in the number even on the same door.
R-value Explained:
R-value is a measure of thermal resistance used in the building and construction industry. Specifically, R-value is thermal resistance to heat flow. The higher the R-value number, the better the insulating properties of the material. However an R-value of 16 does not offer twice as much thermal resistance or twice the energy savings of an R-value of 8. An R-value of 16 offers a 5 percent reduction in heat flow and 5 percent improvement in energy efficiency over an R-value of 8, not 50 percent. No matter the R-Value, an insulated garage door will reduce the amount of heat flow and improve energy efficiency of a home.