Does it just vary when you are driving, or does it change when the car is sitting still? If it just changes when driving, its just because of the fuel sloshing around in the tank, or angle changes such as a hill or something. If it fluxuates while sitting still, the sensor is probably bad, which would probably involve dropping the tank to replace.
I think its normal. My gauge floats too. The gas sloshes around in the tank so the sensor would see it as going up and down. When sitting at a light or when your parked and the car is on thats really how much you have left. I noticed it happens on corners, stopping, accerating, going up hills, and bumps.