I like the steak analogy...
I think that when it comes to retailers like Autozone and Advance, their business models are just more retail oriented than wholesale/commercial. In some markets they try to grow their comercial business but in most cases never fully succeed because of the push of lower cost/inferior product, lack of product knowledge by their staff, higher focus on retail, etc. They know this and it is why a company like Advance has Autopart International, who on the other hand does well in wholesale/commercial like your Napas, Carquests, and other wholesale parts jobbers..etc. You usually get knowledge and more focus towards the wholesale/independent shop.
The other important factor to remember about retailers like Advance or Autozone, they just will never understand your business like you need them to. They don't have the warranties or programs available for independent repair shops. They are retailers who have a business models built for the "DIY" market.
I think the easiest way to see it is ...
Autozone & Advance sell retail and dabble in wholesale.
Napa, Carquest, other parts jobbers sell wholesale and dabble in retail.
If you want to learn more about the major auto part companies, here you go...It's good to know about the company(s) you are buying from...
AutoZone
Advance Auto Parts
Carquest
NAPA
O'Reilly Auto Parts
Checker Auto Parts