I HAVE SEARCHED AND THE BUG MCPE-17564 HAS BEEN CLOSED AS FIXED BUT IT IS NOT.
I have an enchanted diamond sword with Smite V, Fire Aspect II, Looting III, Unbreaking III and Knockback II. When using it against zombies it takes three hits to kill the zombies where as it only takes one to kill skeletons.
I am also playing on a realm if that makes a difference.
Also affects variants such as Zombie Pigman, Zombie Villager, and Husk as reported by MCPE-19924
Identical issue in 1.0.3, smite has no boosting effect. Easily tested in creative; Smite IV, Smite V, and Unenchanted diamond swords take three hits for zombies and husks, while Sharpness IV and V diamond swords take only two hits.