When flying the Classic 747, one must adjust idle power descent point depending on TOD weight if planning a min fuel burn descent without ATC interference. The heavier the weight, the longer the glide, and the lower the weight, the shorter the glide. The difference in a no wind situation glide between landing weights of 630,000# and 380,000# could be ~25-30 NM.
Explain please.
All glider pilots know that best no-wind gliding range is achieved at best L/D and the range is indipendent of weight. Counter to intuition, the heaver aircraft will glide the same distance, but it will do so at a higher speed. That's why racing sailplanes load up with water ballast. They can glide the same distance at higher speeds.
It's not immediately obvious to me why a Classic 747, unrestricted by ATC considerations, would obey a different set of aerodynamic laws. Perhaps you're flying the heavy version at best L/D (or more likely at some speed above best L/D) and comparing that to the light version being flown at the same high speed; i.e. even further above the best L/D for the light version. That would indeed cause the light aircraft to glide a shorter distance. Could that be it?