I asked my doctor about this, because I've noticed that even in normal people, the lip texture looks different on the outer third and inner 2/3rd. My doc (who's super smart) says that the outer third is called the cutaneous region, and the inner two thirds is the transition region, and inside the mouth becomes the mucosal region. And she says the reason that the EC sufferers all have the same area of disease is because of the distinctive traits of a transition region. Most importantly, they are a zone of rapid cell turn-over (even in normal people), so it's very prone to malignancy and becoming diseased. In fact, there are several other "transitional zones" in the body, and they also show high rates of malignancy.
By the way, my lips also used to be fully covered by EC - 100%. But after I went on Monsterlip's no balm no moisturizer plan, the outer third have slowly healed.