Yeah, it's common for their to be a discrepancy between the gauge on the tank and the one in the rifle. The gauges on the tanks are more accurate though, I would always go with what the pressue is reading on the gaguge on the tank. The ones on the rifles are an approximation.
Go with the greater of the two readings this way you are always safe from over filling, If you use a Scuba or paintball shop to fill your tank explain the issue and they can check your tank gauge and tell you how much it is off if any so you will know which one to believe