I'd vote for the drivetrain getting broken in, with possibly an adjustment for seasonal changes and/or fuel formulations. I regularly get 400+ miles from a tank of gas in my car in the summer, but it drops to 325-350 in the winter. I blame increased use of the A/C (for defrost) and am curious if the difference between "summer gas" and "winter gas" contributes in any way.
nate
Also in the summer your oil viscosity will be down. I'd suspect the service station has been using a higher viscosity mineral oil. Don't think it's synthetic that makes the difference, I think its viscosity (at operating temperature). Synthetics hold their viscosity better as they warm up (which goes the "wrong" way).