To show why there is so much water held in the pipe, take a look at the way the plumbing is designed. The grey tank has a very large horizontal surface and since it mostly stops pushing water out the hose after air enters the pump, that's where it stays. As you can see from the schematic, a very large surface in the tank will also hold water if the valve is open. Plus, there's more like 6 to 8 feet of pipe, even if you close the valve.
So, that's why it's more likely to be closer to 2 or 3 gallons or more and why antifreeze in severe winters likely won't (and wasn't) enough to protect the pump.