Usually when the gauge is getting near empty the needle will jump some, take a dive to R, then recover...just gets a little squirelly I guess.
Wierd, though, Supposedly the tank is 18.5 gallons + 3.0 gallon reserve. I take that to mean that when the idiot light comes on you have 3 gallons left. I've run it down to that point but can only top off the tank at around 15 gallons. Does the 18.5 include the 3 gallon reserve?