Hi Hunter owners. I own a Hunter 460 built in 2000. To keep this post short I will cut to the chase. My manual and most all the specifications say the diesel capacity is 100 gallons (US). That's what always thought and planned for. When fixing a recent fuel problem I went to open up the tank and found a label on the top of the tank that said it was an 86 gallon tank. With the tank totally empty and cleaned we took her to the fueldock and sure enough the tank was full at around 86 gallons.
Not about fuel capacity, but I recently replaced the waterheater on our H38. Owners manual states a capacity of 6 gallons. When I took out the old heater and could finally read the label it said 5 gallon. Your story sounds familiar, I think there is the design of the boat and then there is what they build based on parts availability and shop floor constraints.