It could be the fuel recently bought has lower RVP (Reid Vapor Pressure). Lower RVP is mandated Federally for summer fuel (normally shipping 100% by May 1). However I recently found an company report on line that suggests to me that the manufactures don't just switch summer to winter gasoline. The way I read it is they ramp up and then ramp down the high volatile components (like benzene) so in January they are producing the highest RVP fuel, and by March they have already begun to reduce the RVP.