enchanted books

The algorithm for obtaining enchanted books or tools is strange ... broken.

• on the one hand, certain enchants have been very common lately, others rarely or never occur. The wiki has probabilities that I cannot confirm.

• An example: I get it at the enchanting table
  constantly "piercing" and never "multishot", no matter what I do.

• On the other hand I fish Enchanted Book with 3 ... 4 enchants of the maximum level. I recently fished a book with 9 enchants, 8 of them are at maximum.

Is that normal?