Wow that's some difference. Is the air pipe free from twists, kinks etc?
I noticed on mine that the grommet actually was fairly pinching the pipe, this was helpful in my case, as it helped to stop most of the buzz I was getting from the needle.
I used the pipe that was supplied with my gauge, but I noticed a narrower diameter pipe I had fitted on aswell. I would imagine this would affect the reading if I used that.
Did you get much buzzing from the needle in the gauge? I'm sure this wears the mechanism over time.