In probably every version this is the case:
Not every in game mob is on the mob list in 'statistics'
(when pressing "esc" in game => Statistics => mobs)
(The place where it says "****" killed you "" times and you killed "**" "*" times)

The mobs missing in 1.7.9 are:
Witherskeleton
Wither
Enderdragon
Iron golem
Snow Golem

And in 1.8 will probably be added to the list of missing mobs:
Endermite
Guardian
Guardian boss

I think even the enderdragon should be added to the list because a lot of SMP have the end reset now and then.

Thank you for reviewing.
I consider this as a bug because it's a mob list and so should contain every mob.