I'm not really an AFMA expert, but the problem is as such.
lens is -15 and the body is +4 making the total adjustment 19 notches till AF is perfect.
Then you get a new body and the same lens. The lens is still at -15, but the new body is at +9, so now you are at 24 notches till perfect.
And if I'm wrong, please feel free to correct me out there.
So it is really about the lens, because let's say your body is +15 and your lens is +13, then you only have 2 in total difference. So when you sell it to someone without AFMA, their body is -10 and the lens is +13 for a total difference of 23... which could be a real problem.
So my presumption is that you want to make sure that you are identifying whether it is a combination of body + lens which is getting the high number or whether it is the lens itself.