Your absolutely right, the math A+B trace is clipped with the vertical scale set at 1v/div. I did not suspect the clip because the top of the A+B trace appears to be visible (it should not be there is the signal is "off" the screen).
Anyway, I changed the vertical scale of A+B to 2v/div and "magically" the resultant is what I expected to see.
I would have thought there would be some flag/message for the user (me) that the displayed section of a trace was being vertically clipped.
So what did I learn? If the signal touches the top/bottom of the display assume the signal trace is being clipped from the display.