When you select Mid-Price from the TWS drop down, the system locks on to an absolute price according to the current B/A. If it takes you a couple of seconds to click submit and the B/A has moved away, you are still locked into the absolute price although the relative Mid-Price has moved.
By selecting the Adaptive Algo, it will find the relative "mid" when you click submit as shown on the first animation.
In fact the choices are: bid +20% , bid +30%, mid, ask -30%, ask -20%.
Cool. Now, I know how to capture a half-nickel increment. Gravy.