There are no bookcases in the thrown vaults

I began new game in a mode a survival. After a while I decided to make a Enchantment Table and to surround it with bookcases. Without complicating itself life, I decided to find bookcases in thrown vaults, but in one of them I didn't find bookcases.
Using the third-party program for district map display, the Minutor program, I found that in the thrown vaults there are empty square areas very similar to places where "library" (book shelves) could settle down.
Having begun other new game I found the same effect of lack of bookcases in the thrown vaults.