Your ground object is a bullet, and your player object is on top of it. Your ground object is moving to left with X speed and your player object is moving to right in X speed, so it feels like player is stationary.
But when your ground object reaches it's limit, you take it, and put it to the end (right) of itself. When you do this, if your player is touching the ground, it also takes the player to further right, along with itself. I don't even have dropboxaccount so it will be hard for me to send you back the capx. But as far as I know, there is a "wrap" behaviour you can use for the same effect. Suggest you take a look at it.