There are three volatility classes for ethanol (designated 1, 2, and 3). Class 1 is summertime E85, and needs to have a minimum of 79 percent ethanol (so even E85 doesn't have to be E85). Class 2's ethanol minimum is 74. And Class 3, wintertime ethanol, is 70 percent. In some parts of the country, Wyoming, for example, Class 3 is sold from October through May and Class 1 is sold only in July and August. You can find what fuel is sold when in your state in the the Department of Energy's "Handbook for Handling, Storing and Dispensing E85" (which you can download here in PDF form) on pages 22 and 23.
Robert White, Director of Operations of Ethanol Promotion & Information Council (EPIC) responded to a question about E70 from AutoblogGreen and said that, "The winter blend of E85 is seasonal, but is set by geographical regions. The seasonal change in fuel is to help with cold temperatures. The more hydrocarbon in the fuel, the lower the flash point to ignite the fuel when starting. There are no notices at the pump, and if labeled properly, the FCC label reads 'Minimum 70% Ethanol' - the term E85 is a generic term, not an official government term."
The Handbook says about Seasonally Adjusted Blends that:
"The amount of alcohol in the fuel ethanol blend depends on the geographical region and the season. (A complete breakdown by volatility class for the geographical fuel regions can be found in Appendix A). In cold weather, more gasoline is added to the blend to ensure proper starting. A minimum of 70% by volume of alcohol is permitted in the winter blend by the ASTM fuel standard. This seasonal blending from 15% to 30% gasoline limits concerns about winter cold starting and are similar to seasonal adjustments of volatility (vapor pressure) used in gasoline blending throughout the United States."
[Source: Robert White]