REI's web page for the Gregory Whitney backpack lists its volume as
87 / 95 / 103 liter
What do you think this exactly mean? Does the backpack have three different ways to use with these different volumes? If so, any idea how they are different?
The increase in pack height to accommodate the S, M and L torso ranges changes the capacity. You'll note the weight is also different because of the extra fabric needed. The hipbelts usually change size quite a bit also since you apparently can't have a tall torso with a skinny waist.
It looks like those the backpack comes in three sizes, Small, Medium and Large and what you are seeing is the volume for each size.