The first thing I did upon receiving the batteries was to take a detailed look at their physical appearance. There are only so many battery manufactures in the world and I am pretty sure Costco doesn't manufacture their own. I started with checking all the tops of the cells and noticed similarity between the Kirkland Signature and Duracell as you can see in the image below.
Up to this point all of the alkaline batteries I have tested were marked with an expiration date of 2024, however the Kirkland Signature batteries were dated for 2023. So we can assume that the Kirkland Signature batteries were manufactured a year or so before all the others. As alkaline batteries age they lose capacity and thus even two batteries of the same brand will not have the same capacity if one is older than the other.
An additional explanation to the difference in capacity could be that Duracell only uses cells that don't meet spec when making store brand batteries. Either way, they lasted longer than regular Duracell batteries and at a lower cost!