Custom mobs with customized health aren't customized.

I'm making a map and I was customizing a Wither Skeleton to have 4000 Health Points using an NBT editor but when tried battling it with a sharpness X diamond sword, the wither skeleton died in one hit. I tried spawning another one with 4000 Health Points and used a normal diamond sword (without enchantments) and the wither skeleton died in 3 hits (which is the average amount it takes to kill a wither skeleton with a diamond sword). I tried it for other mobs like customizing a pig with 1000 and I still killed the pig in two hits with a normal diamond sword. It convinced me to think that the custom mobs' health points weren't customized. I've done this in previous versions in 1.5 and 1.5.2, but for 1.6 pre-release, it doesn't seem to work. This applies for all gamemodes.