Could be a problem with the flaot. Could be a problem with the sensor. I rarely, if ever go below 1/2 tank. Lately, I've been running between 1/3 and 3/4. I noticed that my guage reads much higher when running between 2K and full on plane. I also found out the hard way that my guage reads about +/- 15 % lower than what is actually in the tank. I then noticed that the inaccurracy only seems to be when the boat is stopped or at headway speeds. Initially, I suspected that it was reading higher when running between 2K and 3800K. I suspected the running attitude was sloshing fuel to the back of the tank and making it read higher than it was. I was incorrect. At rest, it reads lower than what is really there, and when running above 2K is actually reading accurately. (As accurate as a boat fuel guage can read).
Not sure that helps, but it certainly sounds inline with what you're experiencing.