I've posted a few times that I have this problem too - took it to the dealer who couldn't see anything obviously wrong. If I fill up when the little fuel symbol starts blinking (while riding, not off the side stand) I never put in more than 3.75 gallons, and if I wait until the last solid bar also starts to blink, I put in about 4.25 gallons on a 5.3 gallon tank, same as you. For those keeping score at home, that's FOUR quarts of fuel left in the tank on the last blinking bar, versus 2.6!
So now, I just ignore the first blinking fuel symbol and know that when the bar starts blinking I'm down to a gallon. Though if I were to ride somewhere desolate with 75+ miles between gas stations I'd respect that first blinking light a bit more :P
I considered taking it back to the dealer to demand a warranty repair, but I'm not sure this is a problem serious enough to fix - sure it may be covered (or not), but it'd require removing the tank, checking the fuel float, maybe a bunch of other things, etc., and I'd lose a few days or a week of riding time for no good reason.