0
Probably because the difference is that one of them is 3D.
Monster vs Aliens 3D
The Amazing Atomic Monster in 3D - 2000 was released on:
USA: 3 March 2000
It was developed by a man named Malcolm Evans. The game was called 3D Monster Maze.
You Can Play This - 2009 Monster Hunter Portable 3D 3-21 was released on:
USA: 13 October 2011