Diamond Efficiency IV&V Shovels

In the latest snapshot I've noticed a change in behaviour between Diamond shovels with Efficiency IV and V enchants. I don't know if this was intentional or a bug but haven't seen it anyway on the change log. In previous versions these shovels have torn through dirt very quickly, now they behave like Efficiency II and III do.

Steps to reproduce:
1. Create world in version 1.7.10
2. Give self Diamond shovel with Efficiency IV
3. Change to survival mode if not already
4. Dig some dirt to observe the behaviour
5. Reopen world in latest snapshot
6. Dig some dirt to observe the behaviour
7. See the difference.

Like I say, I don't actually know if this is a bug or intentional but I haven't seen it anywhere on the change logs