I didn't know the difference either. The difference is the gun. If you look at the game's marine, his gun isn't as nice as Underpantz's gun. Underpantz's gun looks more like the gun an official Blizzard wallpaper:
Here's a pic of the game's marine (it's the one at the right, not the left)
He added decals to the gun and shield and marine's armor, tweaked colors, changed the texture of the marine's helmet from smooth to a hexagonal pattern and finally made the helmet transparent so you can slightly see the marine's face.