Kirkland is their store brand, but Costco doesn't manufacture anything. What company in the world could process film, bake bread, make facial tissues, and then raise chickens all in the same factory? Kirkland cookies, of all types, for example, are Pilsbury. Kirkland chicken breasts are Tyson. It's the same type of thing that Sears does with the Kenmore series of appliances--those are all for the most part manufactured by Whirlpool.
The reason they are cheap is because Costco carries all of the costs associated with distribution (since they only deal with their warehouses) and buy millions of tons of lava lamps at once.
I for one will be checking out this hot deal this weekend when I go buy some Kirkland (aka Kingsford) Charcoal.