An observation from the BEA preliminary report pages 22-23
From the
previous DUS-BCN flight, aircraft level at 37000ft, captain leaves cockpit, seconds later ATC cleared them firstly to 35000ft and then circa
40 seconds later to 21000ft.
Lots of manipulation of target altitude to to 100ft etc during the time the captain wasn't in the cockpit, but when the captain returned to the cockpit four minutes later the target altitude selected was 25000ft and not 21000ft
ATC
expecting 21000ft within an unstated time frame, but in the absence of terrain clearance issues would there be any reason not to set the desired altitude at anything other than 21000?
No indication of if this setting issue was resolved by subsequent communications and actions by the flight crew and if it raised any queries.