could it be that the computer decided the battery needs to be balanced, fully charged and recalibrated? The car could be working in mysterious ways, but it doesn't have to be quirk or any type of gremlin. The underlying software is rather complicated and it could be doing things which we don't even know about. Even much older, non hybrid cars had for example learning period, where computer would collect data about driver's habits and set the car accordingly, so for the first few miles the car would drive differently, every time you disconnect battery for example. Unless there are error codes I wouldn't worry about it. As far as bad gas is concerned, It could explain lower mpg, but how would that explain engine running more often, instead of in EV mode? and if the engine runs more, the mpg could suffer even with "good" gas.