If you take a look at the correlations between volumes and dimensions of Osprey packs, you might recognize the inconsistencies. Numbers won't add up.
Upon research, I figured that this is because of two reasons.
Reason 1 - Rounding
Let's consider Farpoint 40.
Osprey website lists its specs as follows:
Considering M/L size, 21 x 14 x 9 inches gives 2646 cubic inches, which is equal to 43.36 liters.
However, it's listed as 2441 cubic inches and 40 liters. This inconsistency is seen in most Osprey packs.
The reason behind this inconsistency is rounding up the dimensions. We know that height, width and depth of Farpoint 40 M/L size isn't exactly 21, 14 and 9 inches - but very close.
Let's assume that height of Farpoint M/L size is between 20 - 21 inches, width is between 13-14 inches and depth is between 8-9 inches and calculate the volume for 20H, 13D and 8W.
20 x 13 x 8 inches give 2080 cubic inches, which is equal to 34.09 liters. As you see, the declared volume is between limits of 34.09 liters and 43.36 liters.
As a result, dimensions listed in specs can be rounded up or down.
Reason 2 - Pliable Materials
Let's consider a pack made of relatively more pliable materials, Kestrel 48.
Osprey lists its specs in their website as follows:
Considering M/L size, 28 x 15 x 16 inches gives 6720 cubic inches, which is equal to 110.12 liters! Huge difference.
Decreasing the dimensions by one inch and calculating 27 x 14 x 15 5670 cubic inches, which is equal to 92.91! We're not even close this time.
So I wondered about it but I couldn't come up with anything, so I mailed Osprey Customer Service - both US and EU offices. For the sake of clarity I'll use both answers and try to explain what's going on.
I'm also told that;
They provided me a sketch to help me imagine it easier:
And this is the second reason.
Dimensions of some Osprey packs are rounded up or down.
Dimensions of backpacks with pliable materials aren't as high as their listings since they're the highest possible dimensions and hence their multiplication doesn't make sense in terms of volume.
#Additional note: Customer Service of Osprey is indeed as good as they say! They were super nice to me, provided sketches, replied multiple times very quickly and even offered talking on the phone.