Wither Skeleton Skull drop rate too high

Wither Skeletons have a 2.5% chance of dropping their skulls when killed.
When the Looting enchantment is applied, the chance increases by 2% per level.
As a result, if you have the Looting III enchantment, skulls have an 8.5% chance of dropping.

Intended Behaviour
The chance of dropping should only increase by 1% per level.