My guess is that it is the auto-calibration feature of the PF. Probably when the pedal is switched to the other thing, it appears an an open circuit to the PF, which is then viewed as 100%. As a result, when you switch back to the PF, the lower resistance of the actual pedal only gives you 88%. You could try setting up the switch so that when the PF is not selected, the tip is shorted to ground – this might fix the above problem.
