You can get different power ratings from the same basic engine on an A 320 just as you can on an A 321. It may be that the repat A 320 had low power engines while the Thomas Cook 321 had higher power engines, as it obviously did the trip regularly. The CAA had to find aircraft, and lots of them. at short notice so may not have had a suitable aircraft available at the time. I notice that a Malaysian Airlines A 380 seems to have been used for some flights.