oh right and a question. there are 9 cube parts in the game right? and one of the nine is impossible to grab too?
There are 8 cube bits in the game, and they are all obtainable ;)
There is one which you cannot get by jumping - you have to solve a puzzle to reach it.
If you already got an ending without collecting them that means you found at least one of the *other* main collectible type in the game - Anti-Cubes. Those are separate, and equivalent to a whole cube.