Yeah, it's a mistake in the game too, but I think they did it on purpose. You can download a trailer of the game here:
xboxmovies.teamxbox.com
In the trailer it show Anakin using Force lightning and stuff, which, of course, Lucas has said many times that Anakin couldn't do because of the mechanical arm (and soon to be more). I think they wanted to have cool powers to use in the game so they just made his arm more normal to avoid the contradiciton.
It's right at the end of the trailer, and may not be an actual gameshot, but I'm assuming you can use it since it's in the trailer.