Okay, forget it. I suddenly understand that when you top up the cartridges, it doesn't make any difference to the chip. The system isn't really measuring the real-time content of the cartridges. It simply reporting the last date of the cartridge chip.
A little bit more reading of the Piezography site and I now see that it is a bad mistake to top up the cartridges before you get the Epson low Inc warning on the status page.