This is Celestia we are talking about. She has a weird habit of creating hidden sections of stuff anywhere she stores loot. Thats why the castle library has a restricted section, the public Canterlot library has a restricted section, and do I even need to bring up the Castle of the Two Sisters, the library in that one alone had a secret study with some books including the journal of the two sisters AND a different secret room to store one evil wishing book?  
Honestly, it would surprise me if Celestia doesnt have secret libraries in Twilight’s castle and the Crystal Empire.

Celestia has two restricted sections. A regular restricted section and a restricted restricted section.

@Background Pony #D358  
Castle should be safer than the library, no?

Considering the way Celestia runs things, there are probably more restricted sections than non-restricted ones.

Apparently only one restricted section can exist

Damn…everything would’ve been solved if they’d put the book in the restricted section at the Canterlot library…

Yeah, no continuity problems here because they’re different libraries. And remember that Twilight didn’t even know about the restricted section of the second one.

The top is The restricted section of the Star Swirl wing.  
The bottom is the restricted session of the canterlot library