I believe this is a bug. Feel free to close if this is intended functionality.

Matrix Enchanting (by default) allows 3 pieces to be generated, with extra pieces being granted depending on two factors: bookshelves present at the enchanter, and enchantability of the material being enchanted.

Currently, the number of extra pieces gained from enchantability is set to half the enchantability of the item, or half the number of bookshelves, whichever is lesser:

int enchantabilityCount = (Math.min(bookshelfPower, enchantability)) / 2;

However, the number of bookshelves counted by the enchanter is already limited to the "Max Bookshelves" configuration value (defaults to 15):

bookshelfPower = Math.min((int) power, MatrixEnchantingModule.maxBookshelves);

This means that any material more enchantable than iron (enchantability 14) cannot generate any more pieces than iron can if the player has the maximum 15 bookshelves in place.