Many people report having similar issues, but I don't remember anyone testing it like you did and seeing that kind of discrepancy. Usually, the best solution that follows attempting to clear out any debris, and taking the joysticks apart and placing it all back together, is to replace the actual stick. In your case, I would not know what joystick to replace, or if you should replace both. It may be that it needs an RMA.
I would suggest going through this thread:
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