(I suspect that this is because they advertise flights as lasting longer than they actually do in order to massage their punctuality statistics but the result is the same).
In the days when I was flying on 737s once a week from FFaM to CdG, we all knew that Lufhansa trip times were ten minutes longer in the timetable than Air France times in the timetable. Hence LH was 'always' on time. Had no bearing on with whom we flew as AF offered better cabin service, putting us business class chaps into first, LH put is into cattle class but with metal cutlery and china plates.....
Oh sorry, shouldn't mention cutlery and plates on an FR thread.