Axe Enchanting Not Working Properly

The enchanting system is broken. After going through 23 different diamond axes in survival mode, it mostly outputted enchantments such as bane of arthropods, sharpness, fortune, and smite rather than the more useful enchantments such as efficiency or unbreaking. I then tested it in creative with 27 book shelves and had no better luck. Efficiency only popped up about 20 percent of the time which seems very low.