When the main tank runs out and the engine draws fuel from the header tank only, the mixture will be richer. Why? Because the fuel does not have to travel the long fuel line from the main tank to the header. Only air pressure is in that line now and air has a much lower 'friction' coefficient than liquid. So the engine finds it easier to suck the fuel and the mixture richens. Try it yourself at home. Get a long straw and suck on a coke and then get a short one and suck.

Fritz