Shooting with a bow with strength deals less than normal damage

When shooting with a bow with strength, it deals less than the strength's usual damage.The Wither I could 1-hit w/ my fist, odd enough.

What I expected to happen was...:
One-hit KOing a skeleton w/ strength 255

What actually happened was...:
It took 5 hits and lived.

Steps to Reproduce:
1. /effect (Username) 5 (6000-1000000) 255
2.Get a bow & arrow
3.Shoot
2. ...
3. ...