This is typical because, in your flight history, a flight arrival or departure time was incorrectly entered, for example - departure on August 1 and arrival on August 20 - giving you a flight with a duration of 20 days (which would shift your stats). this type of flight issue may occur after being imported from the connected Inbox, or from TripIt.

You can easily change the duration by editing the trip. See more here how can you edit trips.