The add_meta_box code you’ve posted does certainly register the metabox to the bottom of the side area. If the box is shown in the center then i imagine it’s because you’ve(at some point) moved the metabox, as can be done with any of the metaboxes(all drag and drop). WordPress remembers where you move boxes to, and only adheres to the positioning inside the add_meta_box as a default view(when they’ve not yet been moved).
The solution would be to drag and drop the box back to the side area, assuming this is just a case of re-positioning the metabox.