So as you see on the video , the front side is moving together but the back side is not ! I want to follow a tutorial video but my mirror and the tutorial mirror is behave differently ! But why ? As I see on the tutorial both sides moving together ! Here is the video how my mirror works!
In the video, (having used it before,) the tutor is still using B Box-Select, in the shot you're referring to. In wireframe mode, that will select every vertex, however deep, within the box.
In your video, you are selecting directly, with a right-click. This will only select the vertex closest to the viewing plane.