Its symmetrical. The 2nd half of the path is a mirror opposite of 1st half. So all the twists and turns it takes to get to the 1/2 way point all get redone in mirror opposite on the way back. Thats why this model appears to have no twisting from one end to the other.
But try it on a model that is not symetrical and there will be wtwisting when the end comes back to the beginning.
I've attached a model that shows what I mean. Its hard to how it on screen shots, but I've mnade a few scenes in the model to show it pretty quick I think,
Chris