Essentially, your inherent stoneskin is "cast" at a higher level than normal, which makes it harder to dispel and depending on your level may end up meaning extra duration. At hero specifically, if that's a case that's important to you, the math works out such that you're always getting extra duration.

I probably should stop letting other people write the helpfiles for some of the edges, although I do really appreciate not having to do everything for new edges -- sometimes important clarity or detail is lost in translation.