I'm going to hazard a guess here that it's partly down to how the auto drives, if our 1.4 auto Corsa DS at work are anything to go by, they are shocking to drive, you have to kick the arse out of them to get anywhere, because all they do is rev and not actually pick up much speed, take ages to change up so the revs are unnecessarily high for ages, and even at that, once they are up to speed in the highest gear for that speed they are still at very revs.
My 1.4 petrol manual is starting to even out mpg wise, the trip 1 was showing a 39mpg average over total distance covered since I got it, but the trip 2 which I reset each time I fuel up was showing 43mpg for each time, couldn't figure out how if I was getting 43mpg on every fuel up, the total average wasn't 43 too, now done 1800 miles, my trip one is showing that over that 1800 miles I've got 42mpg, and since my last fill up I've done 275 miles and also averaging 42mpg.
Really impressed tbh as that is all town centre and congested motorway driving 10 miles each way every day to work and back, my 1.2 Clio couldn't beat 35mpg on the same route. In fact it only did 44mpg on a 800mile round trip to Yarmouth and back once at a constant 70mph.