Not necessarily a "lower grade beef". It depends on what the market has on hand. Keep in mind that they can't sell high priced shoulder meat that could better be sold as roast or even now the round is considered higher price. All parts of the younger cattle now are sold as in tact meat. All hamburger comes from aged cull cows that would be too tough as cuts of meat. The fat comes from the trimmings of the younger animals.
My preference for hamburger is the loin meat from a cow mixed with the fat from the rib trimming or other fat off a younger animal. The shoulder meat or rump meat is pretty much equal to that. The reason I like the rib trimmings is that it doesn't render out as quickly and results in a juicier burger. The meat from the round (the hind leg) is the toughest and needs special treatment but is often used in hamburger.
The tougher meats are most used with fat added, from what I know of people who worked in meat packer places, and the tougher meats are considered lower grades? I realize if they have meats that are unsold, they are ground with fat and become a ground beef too.
I think we are saying the same thing, but I have never worked with meat, so my perception is explained from a female shopper view and not an employee.