Counterintuitively, a vertex is *not* just a position in space. A
vertex is a position in x,y,z,u,v,w, and normal space, etc. If two
vertices happen to have the same x,y,x position, but differ in some of
the other dimensions, then they are treated as different vertices.
Imagine trying to convince OpenGL that two vertices that shared X and Y
coordinates but differed in the Z coordinate were actually the same
vertex. Sure, you would hate to duplicate the X and Y data which is
identical between them, but implimenting such a special case isn't
really worth the trouble.