Sounds like the sender has the wrong resistance for the gauge. The gauge should read empty when it's grounded and full when the total resistance to ground meets the specs for the gauge. It sounds like there's not enough resistance to drive the gauge to the full mark.
Here's a
wiring diagram for a 1938 Chevy that might help explain how the system works. This
Army manual has a good explanation of the way the tank unit works.
Regarding the movement, the gauge could move from vibration, especially when you're revving the engine. I guess it depends on what you mean by movement.
The movement of the wiper on the resistor doesn't ignite the fuel because it's so close to ground that there's very little current or voltage inside the tank.
