Arco advertises their cash prices. If you pay with cash, then you'll get that price. If you use a credit card, they'll charge you extra.
Also, gas companies don't have oil refineries everywhere. For example, if the oil refinery is on the east coast, it'll cost more to sell the gas in the west coast because of the transportation costs. Arco's oil refinery is in the west coast, so their prices are cheaper than the other brands.
As for the brands of gasoline, it does matter to a certain extent. Although they are based on the same material (crude oil), the additives they use are different. Some additives might burn better than others.
As I mentioned before, it costs money to transport gas from a far away refinery. What they do is to just refine it into a base gasoline. The refinery can then sell the base gasoline to other gas companies (for example, the Arco refinery might sell their base gas to Chevron). Once the base gasoline is in the truck, they can add their own fuel additives to make the gas their own (or they can do it at the gas station). Thus, Chevron might buy gas from Arco in the west coast, and Arco might buy gas from Chevron in the east coast. You'll notice that gas trucks are no longer labeled with brand names. If you follow a gas truck, for example, it might go from Arco, to Shell, to Chevron, etc.
Check out www.toptiergas.com for some gas companies that are considered top tier.
Personally, I would never use Arco. I have even seen mechanics who tear open engines, look at its condition, and ask the owner if he/she uses Arco gasoline (which is always correct). I stick with Chevron, Shell, and Mobil. I'm a car enthusiast, so I prefer to take care of my car and give it the ability to perform at its best.