Damage bug

Here's what happened. I was testing damage so I could make up a little mob castle as a map. I spawned some zombies to test out the starting gear. turns out they die within 30 hits with a plain wooden sword. I tried to put this into experiment by spawning a creeper and see the difference between a wooden sword and a diamond sword. They are really similar, both doing over 20 hits. I then enchanted a diamond sword with sharpness V and it killed a creeper within 3 shots, the normal damage for killing creepers. I then enchanted a wooden sword with sharpness V and it did the same. 3 shots. Is this intended as an upgrade to mob killing or is this a bug. Thank you.