Creepers no longer seem to have 20 health (10 hearts).
This is easily seen by going into creative or survival and hitting a creeper with an Iron Sword. Since an Iron Sword deals 6 damage (3 hearts), it should take 4 strikes to kill it (3 strikes would leave the mob with 2 health left). However, now 3 strikes is all it takes to take down the creeper.
You can also try it with a Wooden Sword: it should take 5 hits with a Wooden Sword (which deals 4 damage) to kill the creeper, but it only takes 4.