True. The pack date, by law applied to the cases by the processor when the meat is cut and cryo'd, is the only legal date. For that you have to see the cases the meat came in or trust your seller to tell you correctly. Unless you're talking about some dairy products and virtually all infant formulas, sell-by dates can be arbitrary as they are determined by the store or chain with (or without) any criteria they wish. They are free to change them and many do if the meat looks good when it nears the sell-by. That, or the meat is ground and re-packaged with a new sell-by.
You can ask the meat manager if the store has any hard-and-fast criteria for determining sell-bys and see what s/he says. But then, as there are no legal standards or requirements, you have to decide if you trust what you're told.