Cool/Heat: Studebaker Avanti cooling system

[Hawklover]

So today I "begged" the use of my buds lift to effectuate a LOF and the coolant removal from my block and radiator.
So yes, the Avanti holds 18 qts with heater. So I find it "odd" that I was only able to 'capture' three gallons of coolant from the block and radiator? I purchased five gallons of green 50/50 Prestone, knowing that 18qts is about four and one half gallons. Upon "refill" I was only able to replace three and one half gallons, almost what I removed. So where is the six additional qts 'hiding'? I opened up the heater to allow coolant to flow into it, and even opened the petcock on the water manifold to allow "recalcitrant' air to unceremoniously escape. What am I not seeing?
Many thanks in advance for your combined counsel!

[Hawklover]

Received this from a retired mechanic:
I think you missed the the fact that there's still a bit of coolant in the bottom of the block below the block drain petcocks. Also the rad might have a bit of light buildup. I've never put the same back on a drain and refill? If you're installing a new engine and radiator the refill might be close to spec capacity. I have a cool little gadget that connects to a water hose and you attach an air hose put it into the upper or lower hose and blow out the block with air and water. You can also do the rad but I wouldn't use it on the new type aluminum and plastic ones.