After creating a web app that parses the player json stat files I noticed the value for stat.mobKills and the sum of all the values for each stat.killEntity record are not always equal.

For one player mobKills was 6 higher than the sum of killEntity.*
For another player mobKills was 1 lower than killEntity.*

It might have something do with exactly how the mob dies, ie by the players fist/weapon vs being pushed off a cliff or set on fire, or even switching between creative and survival and killing entities in both modes. However I've not had time to investigate exactly what is causing this behavior.