Badly explained by me Sid. What I am trying to say is that when the last drop of fuel is pumped from the main tank itself, the display still shows a few kg of fuel in the main tank display. This is because a small amount of fuel ABOVE the supply tank is actually displayed as main tank contents.
This then is used until we reach your picture.
Really, this is true and not Pprune guff, it's from Airbus
You have assumed that when I said "when the main tank is empty", I meant empty on the gauges