I don't know if it is intended but zombies have more hp when battling against them in comparation to other mobs. I used a diamond sword with sharpness 2 and both creepers and skeletons died when hitting them twice, but zombies were quite different, the first hit does 9 hp damage and the second one 10, resulting in them having 1 hp for a third hit to kill them (They have 20, as skeletons and creepers do) meanwhile skeletons and crepeers's first and second hit did 10 hp, enough to kill them in 2 hits.