So the problem is as follows:

Whenever you change any nbt-tag of a Slime its health reverts to his max health.

Steps to reproduce:

• summon in a slime with at least size 1 (since size 0 dies after one hit)
• use "/entitydata @e[type=Slime] {}" and search for the health tag
• hit it once
  > check again and its health went down
• now change any nbt-tag like CustomName with for example "/entitydata @e[type=Slime] {CustomName:"test"}

  > the command displays correct health but if you test it again with the empty brackets {} it will show you the slimes max health, meaning it reseted

Tested with other mobs (Spider, Skeleton, Pig) and none had this problem