I believe it'* just due to slosh. It can only happen when the tank is not full, and there'* room for the gas to slosh around. At half and below, momentum and centrifical force can cause the gas to climb one side of the tank causing the center to seem low...

I think this behavior is normal. It happens on my car as well. The needle doesn't fluctuate as much when the tank is full since the fuel float has nowhere to go. At lower fuel levels, however, fuel sloshes around the tank and the fuel float bobs up and down, giving all sorts of wacky readings. You will probably notice that turning left while accelerating will suddenly get you a few more gallons in your tank.