Pack with larger dimensions but far less storage space: What gives?

[jlb566]

Looking at the moment for a medium-sized, mid-range pack on the cheaper side of things, and I came across the Jansport Katahdin 50 (29.5 x 14.5 x 9), which at $50 seems like a pretty good deal. Looking a little closer, however, I noticed that the pack seems quite large (both in terms of the pictures and the listed dimensions) for a 50L pack. For reference purposes, I found a 70L pack, the EMS Long Trail 70, that is actually smaller in every dimension (27.5 x 13.5 x 8.5) than the Katahdin, but apparently has 20 more liters of storage space. Trying to wrap my head around this, anyone have an idea as to how this might be possible?

[ogg]

Manufacturers kind of play around with numbers but the inches dimensions are just going to be rough guidelines. Looking at the Jansport, if you multiply the given dimensions in inches you get 3848 cu in., which converts to to 63L. But the Jansport website list the pack as 3051 cu in, which is 50L. EMS lists both a 47L Long Trail and a 70L Long Trail. Their dimensions in inches might be for the small, 47L version. The fact that you are not buying a rectangular box with straps is at least partly where the appearance of a discrepancy comes from.

Edited March 18, 2015 by ogg