I've found the gauge on mine goes back up one bar on a long uphill climb & also after stopping and restarting the engine - guess it has something to do with the position of the sender unit ?
Another thing i noticed is on a steady run, the gauge stays on full bars for at least 100 miles, then drops rapidly; doing the same mileage in varied conditions the gauge is much more 'progressive'.
Still, it livens up a boring journey trying to push the miles before it drops a bar