It sounds like you're describing the lack of a fourchette, which is the part where the labia (vaginal lips) enclose the vagina and meet underneath it. Some surgeries succeed in creating one and some don't; the main reason seems to be that the stress of dilation can put a lot of pressure on that area and destroy the delicate work required to maintain the fourchette. So surgeons don't always try to create one - that may be down to how much tissue they have to work with and how elastic that skin is - and sometimes it doesn't "take" when they do. Basically, for surgeons who are willing to try, it seems like the main issue is the personal variation in the bodies they have to work on. This is also, as others have said, a fairly easy revision later on, when dilation frequency had dropped dramatically and the rest of the area has healed up.
I had a one-stage penile inversion surgery with a minimal fourchette (my outer lips do cover the vagina, but the point where they meet underneath it is not well defined), and I actually did pop a couple of stitches in that area from dilation and walking. It did heal eventually, and without a noticeable scar, but it was painful and awkward for a few weeks.
The short answer, then...
It's dependent on the surgeon's willingness, the patient's body, and the luck/speed of healing. It doesn't seem to be directly related to the method of GRS (penile inversion or not). And some porn stars probably paid for the follow-up revision work required to get it "perfect."
Just a point to clarify the terminology, too, because that makes discussing these things easier - the "dilation hole" is the vagina, while the rest of the structures (inner and outer lips, clitoral hood, etc.) are collectively called the "vulva."
