They can only fill your tank up to 80% capacity, never 100%, because propane needs a little space in the tank in order to turn from a liquid into a gas before exiting the tank.
Regarding 'actual' order quantities, I've heard that 300 gallons gets you into the next pricing tier and a quantity discount. That's why I usually wait until I'm down near 30% on my 500 gallon tank to re-order.
Otherwise, the only other thing I can think of is they will adjust the # of gallons delivered to 60-degrees F. As far as I know, they are factoring in the expansion of the propane due to outside temperature in the price.
Also note that there are 2 types of tanks, if you're shopping. One is a DOT tank that requires an inspection every 7 years. It's a thinner metal tank. The nice part about DOT tanks is they can be transported FULL...
I went with a AMSE rated tank, which requires no inspections... it's a heavier-duty, thicker-walled tank.