That is weird that it does not hold 5 gallons. Have you tried weighing it empty and filled with water to determine this?I filled it to the brim with StarSan from an extra Big Mouth Bubbler that has a valve I installed. Assuming the gallon markers on the BMB are correct, the Slimline Torpedo keg holds 4-3/4 gallons. And, really, about 4-1/2 gallons of beer.
But the weighing trick would be more accurate. I'll try that next time it's empty.
I found about the same volumes. I used a keg to keg transfer and could not fit everything from an old keg into the slimline. I then verified that by using my Megapot with gallon graduations. 5 gallons from the Megapot would not fit in the slimline.