rare in vaults

I created a world where there were 500 ominous (blue) vaults, opening them I could not get books such as: gust of wind 2 and gust of wind 3

I only received 6 books of Gust of Wind 1, is that intended? or is this a bug, if this is intended then why is it so rare

from +/- 400 storages get only 4 books gust of wind 1 this is very rare