So in 2019 15 dozen eggs were 30c cheaper than 5 dozen eggs?

Costco is also a special kind of store, where you pay an annual membership to get wholesale-like pricing.

Costco is a bulk grocer. You pay a yearly fee to shop at Costco, and in return you can get extremely good deals on bulk food and items.

Yeah I understand that, I do shop there often. It took me a few re-reads to understand how relatively similar pricing for 15 dozen and 5 dozen eggs in 2019 at the same store helps to compare inflation measurements.