Bionatre,
Thank you for your detailed reply. From your description, it sounds as if yo have both an overjet ( where the top front teeth are too far forward of the bottom front teeth) and an overbite or deepbite ( where the top front teeth almost hide the bottom front teeth when your bite is closed).
The purpose of the biteplate is to keep your back teeth apart so they can grow downwards to meet each other, thereby reducing your deep bite.
The lower arch is supposed to sit just inside the upper arch. Sometimes this is not possible because the upper arch is too narrow and the lower arch gets pushed backwards. I was speculating that this might apply in your case and that the upper arch is being expanded to leave space for the lower arch to be brought forward with elastics.