After firing up the ol' Google search, I found a couple possible reasons.
The Arco website
claims that they are a "high-volume, low-cost producer" and sell approx. twice as much gas per station as other companies. A good reason, but there is more...
Answers.com
suggests that Arco reduces prices reducing "frills." Among which include not accepting credit cards at their stations and adding a $0.35 fee for debit card use.
As cyrnel said... gas is gas. Different straw, same cup.