i don't know for sure, but it seems the gauge, or more accurately the float, could be in a position that is more susceptible to "bounce" based on fuel level.
since the float follows a radial path as the fuel level comes up, maybe it's just in a more vulnerable spot as the fuel splashes around when you're at 3/4 of a tank then if you're at full or 1/4 tank.
just a thought...
