It sounds to me like the gauge might be faulty. I had a similar issue with my first boost gauge so I picked up another (after checking obvious things like the boost line) and the new gauge matched my Accessport readings.
The ECU sensor should be very accurate and if you aren't feeling a loss of power from the butt dyno, I think it definitely points to the gauge. 12 PSI vs. 18 would be enough of a difference to feel IMHO.
If you are able to swap it out easily, it might be worth picking up another gauge at an Autozone or something. If for some reason that didn't work or if you wanted a different type of gauge, you could always return it.
Last edited by jackal8788; 12-05-2014 at 05:11 AM..