[PnP] Water and fluid weight in PnP

[Graeme Chaple]

Does anyone know why water and other fluids (beer, ale wine) are listed 
as having a weight of 1.5 lbs in PnP per quart. If I remember correctly 
a gallon (American measure) of water normally weighs 8.3 lbs or so. 
Thus a quart of water would be just over 2 lbs.   I know this doesn't 
seem like a lot but the weight difference of a Skin IV for example 
would be over 7.5 pounds of weight (22.5 lbs vs 35 lbs.)

I figured I'd ask, it's always made me wonder.
Graeme Chaple