Level 30 enchanting can give lower enchantments such as unbreaking I

Using an enchanting table at maximum level 30 can give you enchantments at lower level than expected. On the screenshot, the shovel is proposed to be enchanted with Unbreaking I. When enchanted, the result was Fortune II, Efficiency III, Unbreaking I, while at this level, the expected result would be Fortune III, Efficiency IV, Unbreaking III instead.