[mdoyle]

The hours are stored on a non-volatile eeprom that either has a capacitor back-up (unlikely) or static ram type storage. Even though these aren't supposed to lose their memory they sometimes do.

I imagine the odometer system uses some type of back-up application where the value is stored and then compared on boot up against the last known value. The hours might not get the same fail-safe treatment since they're not deemed as critical to valuation (although they should be).