Looking at the fuel system schematic diagram again, I am struck by the apparent absence of a means to differentially fill the supply tanks. When there is fuel in the main tank, this isnt necessary, as the transfer pump(s) supply fuel to each supply tank faster than the engines can use it, so the supply tanks fill to the overflow, keeping them topped up, and the surplus fuel drains back into the main tank. Similarly, any fuel which "spills" out of the supply tanks via the overflow is replenished.
However, the diagram shows a single pipe feeding both tanks, which would suggest that any fuel being supplied from the main tank will be shared more or less equally with each supply tank. Fine while there is fuel in the main tank. Once the main tank has been exhausted,(as IMHO it was here) the supply tank levels begin to fall below full. Fuel spillage back into the main tank during manoeuvering would not necessarily be equal from each tank, depending on the manoeuvers, but the return feed from the main tank (assuming the transfer pumps were still on) would be. The resultant assymetry (beyond the designed assymetry) in supply tank contents would have the effect of altering the gap between each supply tank running out. Depending on the combination of manoeuvers, it could lengthen the gap, or shorten it....