Well, it still does not explain why it changes...All a triangle is is three vertecies, so you can multiply all the values above by three to get the vertex info...
No, that's incorrect. It may be correct for the code example you gave later, but that's not automatically how it works. You don't automatically have to create three new vertices to create a triangle. You can share 1, 2 or even three previously created vertices, thus the shape of mesh you create will determine whether you have many vertices or relatively few. A long thin flat plane would have many more vertices than a sphere with the same number of triangles, for example, because the sphere shares more vertices.
Like the others, I've never experienced a fluctuating maximum number of vertices. It's always been 64k for me. I can only assume that a recent updatate ( perhaps to fix one of the recent incompatibility issues ) has interfered with this somehow.