Think about it this way: The monks were the scribes...the printing press of the time period. Since they were religious, they would change anything they considered not worthy of attention by readers--current or future. This means all dirty jokes, anything vulgar or obscene, and anything they considered too common. So, since they were the recorders of the oral literature of the time (since so many were unable to read and write for themselves, the songs, poems, stories, riddles, jokes, etc. were recorded by the monks), they chose what to write down and keep for posterity and what they would ignore. Much was lost because of this, and it stands to reason that of the written literature we do have, much was changed.