OK... had to look it up
A primary difference between winter- and summer-blend gas is their Reid Vapor Pressure (RVP). RVP is a measure of how easily the fuel evaporates at increased temperatures.
The higher the fuel’s RVP, the more easily it evaporates.
Winter-blend fuel requires increased RVP. If the fuel doesn’t evaporate readily in cold temperatures, the engine will start hard and run rough when it’s cold outside.
To achieve this, refiners often blend winter gasoline with butane, a relatively inexpensive additive with a high RVP. They adjust the RVP of the final formulation to as high as 15.0 psi to help the gasoline ignite readily in the cold.
Once the temperature warms up in summer, however, high-RVP gasoline can volatilize more easily, which contributes to increased emissions and air pollution.
For that reason, federal law restricts sales of gasoline with an RVP greater than 9.0 from June 1 through Sept. 15.
To comply, refiners reduce the amount of butane in the gasoline and instead use pricier additives, hence one reason gas prices generally increase in summer. The blending process also takes longer, adding to the cost.
Except Cali.... which has a 7.0 RVP requirement