Sorry, but I can't help you there...... I have noticed that sort of thing with my Roland EV-5 expression pedal a little bit too, I use mine with an XV-88 so I wouldn't know if there was a way to calibrate the pedal on the XP. I don't believe there is on the XV.... for what that's worth.
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John 3:16 For God so loved the world, that He gave His only Son, that whoever believes in Him should not perish, but have everlasting life.