I don't think Costco has 'supply issues' and didn't mean to imply that. I do know that they purchase and supply some items nationally (Kirkland items, items featured in the coupon mailers and such) and other items are purchased regionally (like produce and meats). I'm sure that can affect the cost of some of the goods. I'm sure the manufacturer can also negotiate a retail price with them as well. Remember, a lot of companies do special packaging that is only available at Costco (ever bought Pilot G2 pens or Sharpie markers there?) and I'm sure the pricing is part of those agreements.
I have 3 Costcos within roughly the same distance of my house, and they do not carry all of the same items. They are stocked demographically by ethnicity (I'm in Southern California). One of the stores is in a very Asian area and another is in a more balanced demo. I haven't been in the 3rd one, just got gas there but I want to go check it out and see what their demo is like.
Oddly enough, one of the best Costcos is the one in Chico, Ca. They seem to have a large and varied selection of wines and cheeses that I don't see in other stores. Odd population for that store. It's on the same block as the Sierra Nevada brewery, so it carries all their beers. It's in a college town, so it caters to professors (liberals?) but its a very Republican county. Their stock seems above their demographic somehow.
The new grocery store ads come out today and tomorrow, and I need to go to Costco this week to have my tires rotated and refill two ink cartridges, so I'll do a good comparison of the meat and produce prices for this week and let you know what I find.