The inventory size is 1 greater than the contents length. This is only the case for the inventories of the blocks smithing_table, loom, grindstone, cartography_table, anvil and stonecutter.

The attachment contains a reproduction plugin as requested here "https://www.spigotmc.org/threads/how-to-make-a-bug-report.330701/". Simply click on a slot in one of the relevant inventories and look at the values.