In your picture, it implies that once the main tank is above the "fence" fuel spills into the supplies, it doesn't. The tanks are separated with overflows back to the main. Airbus have issued an updated schematic in which it all makes sense. This is where we had it explained about the metered fuel. The supply tank actually holds more than 90kg, it is DISPLAYED as 47/43 (ish) and a bit is added to the main contents as well.
Really, it IS fact