2 Medium Armour & HP Classes
I recall reading that engineer was at an early point planned to be a heavy armor profession, but was then moved to medium armor category. After that, I suppose they just didn’t see it fit to have engi or ranger belong in the top tier of hp pools.
In the end, Anet never promised a perfect symmetry in these things.
With the introduction of revenant, both heavy and light armour categories will each have a class at low, medium & high base vitality (Ele,Guard; Mesmer,Rev; Warrior,necro respectively).
Medium category is the oddball. Maybe the rebalancing with the new specs will change that? i don’t know which of the two classes would be suitable for high base vit, though. Engies are already pretty kitten tanky & rangers have their pet to take hits for them; either would seem like overkill.
Actually, you could make the argument that since rangers have a permanent, healable, tanky pet, they actually are a high vit class, just masquerading as a medium.