Normally this is because your tank could not hold all the gallons you ordered. All heating oil tanks have a “usable capacity” which is less than the actual capacity. For example a 275 gallon tank has a usable capacity of about 256 gallons. The remaining space is required during the filling process to avoid accidental spills as well as allowing for expansion of the fuel as it warms. During the filling process, the driver listens to your “fill alarm” which tells the driver when your tank is filled to its usable capacity. For safety and to avoid environmental contamination, your driver must stop pumping oil when the vent alarm indicates your tank to be full.