What Reivaxe is saying is that the 4 heads' proportions are not standardized. The most noticeable discrepancy are the ears, Hroth ears' placement/ear size + shape vary between the 4 heads. If you plopped the heads into a 3D program and imposed them on top of each other the point where they meet the head would not line up. Thats what "uniqueness" they are talking about.
I don't have maya on this computer anymore so apologies for less technical examples
Imagine blue lines are standardized features and red are unique features
Every other race has a fixed place where its [non human feature] meets the head, be it horns or animal ears. The heads are uniform,
the faces are more like face plates on nendos. If I were modeling a hairstyle for male Miqo'te, I would only need 1 male Miqote model because options 1, 2, 3 and 4 are uniform. It wouldn't matter if a user played the old man face or the Graha face, their heads + ears are standardized so hair will line up the same on their head regardless.
In comparison, Hrothgar are more unique than they are standardized. Some ears have wider bases, some sit higher on their head, etc. Its more difficult to make a one size fits all hairstyle across their 4 faces, which is the how/why we got earless Hroth hair: they went for a simple one size fits all solution...which as we've all seen, doesn't really look good.