Some of what you're experiencing is very likely variations in wind speed and direction. Also, sometimes what seems like a downhill slope can really be an uphill slope - optical illusion caused by the surrounding terrain. Especially at night.

However, you'll notice when you drive more just how much an effect a head or tailwind can have on your FE. It's the same in a standard car, but without the eyeing of a FE gague, you probably would never have noticed it.