normally petrol meter are equipped with a bimetallic strip that gets heat up when operative, it get itself on/off thru the bimetallic strip which is being heated on operation. the reason is petrol in tank shakes up petrol gauge causing it to relay different readings, which is happening to your car. i guess if it is present in your meter, then your meter is wrongly connected etiher or it is not wokring properly.
for reference, check this.
HowStuffWorks "How Fuel Gauges Work"