What do you say to a guard? Your theory about how the thing he's guarding isn't even real. Yeah, you have this whole thing figured out. You have for a while. At this point, it's only fair that you tell your theory to the guard who's wasting his time.
The guard listens to your theory but then he says his OWN! He thinks there are two sets (せつ) of guarded items, and he's only guarding one. According to his theory, either one could be real! His theory is more plausible, don't you think?