Strength not functioning properly

Strength does not function properly. This can be proved by trying to kill a zombie pigman. They have 20HP, just like most mobs. However, if you drink a strength potion, with a sharpness 3 diamond sword, you should be able to deal 23 damage and kill the pigman in a single blow. However, this is not what happens. I've experimented with this on creative mode, and the pigman needs another hit to die. The only way possible to kill a pigman melee is to get a sharpness 4 diamond sword and then do a critical. Or you can get a power 5 bow and kill it that way. I've even tried with strength 2, which still wont kill the pigman with a 10 dmg sword. Please fix this.