O.C. Costco Wine Selection
I cannot help you directly, but each Costco wine department manager must qualify for several levels of wine list. Often, they get to start at about 8, out of 10, but then must produce, to keep that, or maybe climb to a level 9. Most will never see a level 10. So, it depends. store # 131 might have a level 8 wine list, and then, 6 mos. later, fall to level 7, or maybe climb to level 9.
We have about 12 Costco stores in the Phoenix-Metro Area. Only one is at the top (though I would give it only a level 9+, compared to many CA stores), and that is Scottsdale. I am fortunate to live near the Paradise Valley store, and they are at about a level 8+ - 9. Until they opened, the closest was Desert Ridge. When they opened, they were about level 9-, just below Scottsdale. Though the wine manager (and I, as a customer) tried, they slipped to about level 8. The last time that I was there, they were at about level 6, and of no use to me. They even had to discontinue our house, Summer white, but they had stacks of really cheap whites, that went to the ceiling.
In the end, it is all about the demographics, and the $'s spent on good, to fine wines, at THAT particular store. It could be in a "high-rent" area, with zero winos, and only be level 6, or could be in a more "working-class" area, but one with fine-wine aficionados, who spend the $'s.
Wish that I could help you specifically, but live in the PHX area, an seldom shop at CA Costco stores. Someone CAN help you, as Costco IS popular, and the CA folk know their wines.