What it should be doing is line 9 minus line 10a.  If there's nothing on line 10a then it will look like it's just using line 9.  If you have a case where it's using line 9 without subtracting 10a then you have an "unexpected behavior".

Line 10b and Schedule A never exist in the same universe.  So if you have something on 10b you are not filing Sch A so it doesn't matter how much the medical expenses you're not deducting are being reduced.

Rick