The matchmaking system finds people who are approximately on the same level as you are (+/- 5 levels). All players above level 51 are treated as level 51 thieves. When there are no more players available in this range during skipping (they are all under shield, online or already skipped), the game starts to show fake dungeons based on real ones. The gems and gold in these dungeons are fake. The owner of the original dungeon will not be notified and will not lose anything due to attacks.
This is to prevent you from having no one to attack for a while.