Myth, those are my thoughts exactly. Even their indoor duties would be none too clean. I suspect many a monk back in the day got ink on their sleeves when creating documents, not to mention having to clean the monastery's interior. I suppose it is possible that they have several sets of robes. As someone else pointed out this was a very wealthy monastery. All of the Anglo-Saxon kingdoms were pretty wealthy at this time, and a lot of that wealth was tied up in the churches and monasteries.