Why is the number of tracked aircraft in a period sometimes less than the number of in-service aircraft for that period?

Andrew

Aircraft status is manually set by our data research team vs tracked utilisation that is automatically updated. It’s possible that an aircraft has not been tracked for a period but has not yet had its status updated (to Storage) by the data research team.
The ADS-B transponder is not operating for some reason. This is highly unlikely for commercial aircraft.
We’ve not linked the aircraft’s hex code (transmitted via the ADS-B transponder) to an aircraft’s registration, i.e. we have been unable to identify the exact aircraft being tracked.