Amethyst Clusters interact differently with fortune

Summary: Fully grown amethyst clusters will always drop the maximum amount of shards when mined with fortune.

Steps to reproduce:

1. Enchant any pickaxe with fortune 1, 2 or 3.
2. Mine an amethyst cluster with said pickaxe.

Expected result:

Fortune I should yield 4-5 shards, Fortune II 4-6 shards, and Fortune III 4-7 shards

Observed result:

Fortune I always yield 5 shards, Fortune II 6 shards, and Fortune III 7 shards.