Alright (on the other reasons).
If it's not interfering in any way and if it is not as noticeable as it was in the photos, and since scrubbing with softer-than-Moh's-scale-7 materials didn't remove the circle, it could just be a factory defect with the glass,
albeit potentially not a serious one. There's a chance, however, that the screen may not be homogeneous - hence dropping the phone may break the screen more easily if that circle is a weak spot.
These are all just my speculations. If the circle bothers you, have your L830 replaced (if it's still covered by some warranty). If it doesn't, let it be.
