Efficiency shovels don't break grass as fast as they should

Shovels enchanted with efficiency will not break grass as fast as they should in SMP. Haven't tested in singleplayer

What I expected to happen was...:
I expected the shovel (Diamond, efficiency III) to break the grass as fast as the dirt (pretty much instantly).

What actually happened was...:
The grass broke much more slowly. I crafted a new diamond shovel, didn't enchant it, and found that the shovels broke the grass in the same time

Steps to Reproduce:
1. Enchant diamond shovel with Efficiency III
2. Break grass