mine does this too, i dont have the luxury of a gallons used meter in my cluster but i do track mileage with the trip meter. I am going to try hitting it with a 2x4 next chance I get.
I was hoping several months ago when I added techron to my fuel that it would've been detergent enough to wash away the congealed fuel or whatever it is that has clogged the sender.
A guy at autozone showed me the sending unit out of one of the assemblies and its a very simple design just a little metal wire arm about the gauge of a paperclip that lifts and lowers depending on how much fuel is in the tank (i think its connected to a float ball of some sort) and he said that the ethanol fuel gums it up and causes it to malfunction,
so in essence as Jcat described, since the ethanol fuel causes the sender to react you get a faulty reading at the instrument panel