Originally Posted by creber

Another possible explanation is that the previous day the flight was operated by A330 and not by A340. Which means the crew could come either from the A32S+A330 group or the A330+A340 group. So if the pilot came from the A32S+A330 group, he would have the rating for both the A330 (for the ZRH-TLV flight) and the A321 (for next day's A321 flight back). The co-pilot may have belonged to the A330+A340 group - not a problem from the outset, because he was supposed to fly back on an A330 the next day. But then LX switched to an A321, and he couldn't fly that plane anymore because he only has the rating for the A330+A340.