No need a military guru to explain the difference.
Military field pack are made to the requirements of the armed
forces that requested them.
As a military organisation, they would want the most economical
solution to equip their soldiers at the same time want a bag that
solves their problem.
The military have a stringent requirement to which the manufacturer
have to adhere,
namely,
1. flammability of material
2. durability
3. strength of stitches
4. comfort.
5. quality
Basically they want the best for the cheapest. This would be
unheard of in the commercial world.
Please do take into account that first release of any military
equipment are prone to defects. Just like when trial battalions
receive their LBV and larger field packs, after few months... most
soldiers experience torn zippers or broken buckles.
such problems are usually solved in subsequent releases.
Commercially available bags (Deuter, North Face, Mammut, etc)
Here you are exposed to a large variety of prices, quality,
designs, etc.
But in general, they are more comfortable than military issues
stuff as they are more padded in areas where the military might not
want as they may hinder operational requirments.
Do remember that most of these bags are OEM made. And quality tend
not to differ much between brands of the same class (don't compare
a $400 Arctyrex bag to a $40 Jansport).