I don't think there is anything special going on here, if the 1 nanoamp is multiplied by the 2000 volts we get a very small .000002      watts that this runs at, that multiplied by the hours it has been running (~1541760 hours assuming 8760 hours in a year and disregarding leap years) will give the amount of energy that has been taken from the battery, which is only around 3.1 watt hours. The nominal watt hour rating of an AA alkaline cell at 50ma is 3.9 watt hours (wikipedia)