You're getting the behaviour you want, but not what you expect. When you're moving the camera to the left, you're expecting to see the target move out of view. Due to the fact that it's automatically targeting your object, it's constantly turning toward it as it moves further away, giving the sense of 'zooming out'. Your z is never changing, which the trace reflects, it simply looks like it changes because you are rotating toward the object while translating along the x. Being used to forward-backward being a z translation, it's easy to get a little muddled up in these situations.
A little hard to explain without diagrams or visuals, let me know if this makes sense for you.
