It's all about the additive package, not the bulk fuel, that determines whether it is a Tier 1 fuel. Many additives are added as the truck is loaded, some of the independents do it on delivery site. Mind you, this isn't where the octane rating is determined. That happens during the cracking process.
It's also worthwhile to understand that not all Tier 1 fuels are created equal. The Tier 1 rating means the fuel meets a minimum high standard. Some brands, including Costco, go beyond that minimum requirement.