Wither effect duration is 40 seconds instead of 10 in Normal difficulty

When hit with a skull fired from the Wither, I will get the Wither II effect for 40 seconds. In Normal mode, the effect is supposed to last for only 10 seconds.

Steps to Reproduce:

1. Spawn the Wither in a survival world on Normal mode
2. Get hit by a skull that the Wither shoots

Observed Results:
Wither II status effect has a duration of 40 seconds

Expected Results:
Wither II effect should have a duration of 10 seconds