Yeah, staves are perculiar like that, their damage isn't a straight addition like Royal Crossbow is. It's in its own world. Also, the damage being capped by off hand is slightly incorrect, the damage is capped ON the off hand by the tier, not BY the off hand. The main hand part of the damage is completely unaffected by the tier of the main hand, ability or auto attack. (The difference between each tiers of 10 in terms of damage is only roughly 3-4%, the damage difference between 10 tiers at 70-90 region is at least 10%). I am going with that there is a bug in there somewhere, but whether the bug is with the orb (which is good), or the wand (which is very bad).