There are a few explanations of why the billing counters may show as negative:

1. If the printer IP address has been changed.
2. Out of date device Firmware.
3. If the printer motherboard or controller has been changed and the counters and serial number were not added from the service technician.
4. If MIB's used to collect counters in the printer have changed.
5. Also for some printers, the counter values are only saved if more than 9 pages are printed. Thus you will see sometimes see small negative numbers for printers with very low volume.