Billy, here's your answer: It isn't random, regional, or anything else. The price on Costco rotisserie chicken has to do with the weight of the bird. When it weighs below a certain number, it is priced lower. When it is above a certain weight, it is priced at $4.99, regardless. They do that to control the sales price, and so you get fair value for your money, because chickens weigh different amounts.
If every chicken was priced at $4.99, and the one you select, for example, happens to be a half pound lighter than the one next to it, you just paid more per-pound for the product you received. You'd feel like you were not getting treated fairly. By setting a limit (and I don't know the weight they choose), that everything over that weight sells for $4.99, Costco can say they sell a rotisserie chicken for $4.99. If you want a smaller sized bird than that, look for prices lower than $4.99 - that will give you a lighter bird, sold at a fair per-pound price. But if you want to maximize your savings, look for the heaviest bird you can with a $4.99 price tag. The weight is on the label. If all you see offered is the lower-priced birds, it means the heavier ones have already been sold. Check with them to see if they have a new batch coming out soon - chances are they will, since they refresh the selection all day long. Take a lap or two around the warehouse and get your other purchases, and by the time you get back to the deli, the new chickens will probably be out there.
What Costco tries to do with lighter birds is use them in products they make, like their deli chicken pot pie. But sometimes they get shipments with enough lighter birds that they can't do that, and they end up on the rotisserie chicken sales shelf. You have options.
Dave