Rather bizarre. I assume it's running fine? Does it smell rich, exhaust tips real sooty? Has the bike been in winter storage and you just got her out? Recheck the exhaust to make sure no critter nests are in there causing a restriction. Same with the air filter, recheck it for blockage. Brake dragging? Tire pressure? Low compression would effect mileage as well but I doubt that's an issue.
I would recheck the plug and coil wires.
Could it be additives to the fuel in your area?
