http://www.epa.gov/oms/boutique.htm
Twelve states have adopted their own clean fuel programs for part or all of the state. Most of these states require gasoline with lower volatility than federal standards, and most are effective for only part of the year. These state fuel programs make up eight different kinds of fuels. The federal programs (Reformulated Gasoline and low Reid Vapor Pressure) make up four different kinds of fuel. The combination of federal fuel programs and states' abilities to adopt state fuel controls is intended to reflect a balance that allows areas sufficient flexibility to accomplish air quality needs.
I see 7 types of fuel, plus a low sulfur diesel, listed here:
http://www.epa.gov/o...boutique-list.htm
As to why there shouldn't be a single national standard, it's simple - The fuel standards are based on air quality rules, and the ability of a locality to meet those rules depend on the local environment. These additives aren't free, and can cost in terms of mileage, so requiring everyone to use them doesn't make sense for lots of reasons.
HTH.
Cheers,
Scott.