Spider HealF tag decreases on losing Health

The HealF tag of a Spider decreases with it's Health.

Steps to reproduce:

1.Spawn a Spider (In my case I was using a Spawn Egg and the Spider and I were the only two entities in a Superflat world)

2.Use the command "/entitydata @e {}" to display the Spiders NBT tags in the chat and then compare the "Health" (actual Health the mob is at) and "HealF" tag (maximum Health of the mob, should not change)

3.Punch the Spider (without a weapon)

4.Repeat from Step 2.

To see how it should work try the same experiment with a zombie.

EDIT: Well it indeed happens to the zombie too, but only when it deals damage from burning in sunlight (not burning in general)
Same as Spiders for Creepers... (Did not continue testing then because the problem is all the same)

~tobsam