Possibly because, as one manufacturer does (I think it's Epson, but I'm not 100% sure), your printer keeps track of the number of pages printed with the cartridge and stores that count on the cartridge itself. When the printer software queries the cartridge for a page count, it thinks the cartridge is empty (after all you have printed the maximum number of pages allowed) and therefore shows the cartridge as being empty. As long as you can still print, don't worry too much about it.