Reddit users who claimed to be Costco employees explained that this was because the chickens weighed less than Costco's standard for the $4.99 price tag.
As one Reddit user put it, "I currently work in the service deli and we get sent chickens of all random weights and sizes. Costco has a weight standard for the [$]4.99 price…"
Costco's weight standard is 3 pounds, including the bones, and if the chicken doesn't reach this weight, it gets sold at a lower price or used in Costco's ready-to-eat meals.