Minecraft wiki entry for Enchanting_mechanics, associated text: The first modifier is based on the item's "enchantability," which depends on the material and the type of the item (see the table below). Other enchantable items such as books, bows, crossbows, tridents, and fishing rods have an enchantability of 1 for this purpose. Minecraft picks a number between 0 and half the enchantability, then adds that number plus one to the enchantment level. This random value follows a triangular distribution (like rolling a pair of dice and adding) so results close to a quarter of the enchantability are much more likely than results at the extremes.