@hikermor @OldGuyot 

I wanted to add a couple more details to @REI-CarterC's response:

• The gear lab at REI uses the ASTM (American Society of Test Methods) F2153 - Standard Test Method for Measurement of Backpack Capacity. Take a look at more info here.
• That test specifically 'does not take into consideration areas of the backpack that are not completely enclosed by fabric such as mesh pockets, water bottle holders, and compressor pockets.'
• The process for measuring volume in a pack in this way is to pack the backpack with 20mm plastic non-compressible spheres, then pour them out and measure the volume of spheres. 20mm is about half the size of a ping pong ball, for reference.

This was a really interesting journey into the details of how pack volume and naming come in to play. Thanks for the great discussion!