So, it's a standard large ring 98. Since the charger hump hasn't been removed, I'd look at the rear base. There is a radius at the back of the charger hump that will interfere with the base, lifting the front. Run a level or flat over the tops of the bases to see if they are on the same plane and/ or level. You might find that the rear base is tilted upward at the front edge where it contacts the charger hump. The base may not be contacting the hump but, it may be higher than the front base simply due to the fact that these actions were not designed with scope mounting in mind. Thus, there tends to be a good deal of variation among the various manufacturers with regards to the rear bridge dimensions. Unaltered front rings tend to be much more consistent in size. Bear in mind too that bases tend to be made to fit commercial Mauser actions. When milsurp actions are altered to a commercial configuration the rear bridge is often not the same height as its commercial counterpart. Many folks address this by shimming the bases until they are level. I like to alter the base(s) or make my own as required.
