Does meat/poultry keep in the fridge for just a couple of days after purchase, or a couple of days after the sell-by/use-by date?
The advice you see on sites is for poultry which doesn't come with an expiry date. It comes from the time when meat wasn't sold in individual containers with a date stamped on them. You buy poultry at the butcher, he wrapps it in wax paper, and you can keep that in the fridge for two days. 

If you buy your meat in the supermarket, it does have a date, and that's what you should go by. If it is a "use by" date, then it is clear - this is the last day on which it is safe to eat it. If it is a "sell by" date, you can again turn to the advice you found. It is calculated conservatively, as is usual in food safety, and gives you a margin which is safe even for meat sold after lying around for a few days at the butcher's. If you buy the meat before the "sell by" date, you "get" extra time to use it. 


  raw poultry/meat will degrade much more slowly while on grocery store shelves 


While the system above would work without the need for such a mechanism, there is indeed a difference which goes in that direction. The supermarket meat is packaged under special atmosphere, which is low in pathogens and also differs from normal air in a way which inhibits bacterial growth. It is not sterile, and it definitely goes bad. But still, when compared to the butcher's meat, its total lifetime from slaughter to expiry date is longer. 

This tidbit is not relevant to your practical question though, since this has already been accounted for in the choice of the "use by" or "sell by" date stamped on the package. I am simply sharing it for more background.