I've managed to confirm that the issue does only occur with incremental backups. I haven't had the problem when doing a full volume copy. Obviously it isn't ideal to have the NAS copy all the contents from Volume_1 to Volume_2 every night, or every week for that matter.
The letters it chooses are the first letters of the names of the folders it's supposed to copy. For example, on Volume_1 I have folders Data, Media and Work, and when the incremental backups happens, it creates folders D, M and W. All these folders are empty and nothing is copied to them ever. Even if I let the incremental backup run again the next night, nothing happens.