Ender-chests drop 8 obsidian when broken with a silk touch pickaxe

If you break an ender-chest with a silk touch enchanted pickaxe, it will drop 8 obsidian in addition to 1 ender-chest. It seems that Minecraft is registering, when you break the ender-chest with the pickaxe, both silk touch (dropping the ender-chest) and non-silk touch (dropping the 8 obsidian) simultaneously.

Sometimes it just drops the obsidian as well.