Suddenly I have a lot of miles or hours in my stats

This usually happens because the flight arrival or departure time is incorrectly entered in the flight history, e.g., departing on August 1 and arriving on August 20, which gives you a flight with 20 days duration (which shifts your statistics). This problem with a flight can occur after importing from a connected Inbox program or TripIt.

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