Gamebryo gives you the source to the engine and lets you substitute your own components at will. Mythic worked this way. I'm guessing that Bethesda has coded more and more of their own components over time, and have gradually reached the point where it's their engine, not Gamebryo anymore.
New iteration of their engine you could say, with gamebryo(the rendering engine) replaced with their own. But all artistical improvements are all purely due to the new push in their art department. Edit: On a sidenote, I'm willing to bet that most of the modding community consists of people who are heavily into secondlife…
from Kotaku "Elder Scrolls V: Skyrim's new engine is capable of creating actual human faces. And as you can see above, rather attractive ones at that." Oh so the Engine makes the faces, all this time I thought it was the character artists! That's some primo game journalism there.
Overall the engine has sloppy coding put into it, have you used the editor? Not the most pleasant tool to learn, also the amounts of crashes I had, well I say when it comes to crashes Oblivion/Morrowind was fine for me, but Fallout 3 and New Vegas both really buggy, I've played other games using this engine but Bethesda…