Its the winter versus summer argument. In winter its colder so the air is more dense. Turbo boosts to same pressure but because the charge is cooler it physically has more oxygen so the ECU has to put more fuel in. Opposite is true in summer. Over the last year my mpg dropped from 30 in the summer to 26 during the winter months. Unforunately i then got it re-mapped so has only gone back up to 28 this summer.

And no i am not making it up!!!!!