could be MAF related, the sensor causing the engine to over fuel at low down revs? try disconnecting the MAF sensor (the one in line with air intake) and see if it still happens, that will rule out the MAF sensor at least.
The MAF is brand (and it's an Alfa part). Also this disconnecting the MAF thing...I've never got it to work. The car won't rev if it's disconnected, in fact I've never got it to idle while disconnected.
Thinking about it, it's probably the fuel mixture enriching in anticipation of the engine being breenged
Reving it higher gives the engine time to burn off the excess fuel, while reving lower, well, it's not worth over-fueling
My mpg is very good for a 2litre engine so not not worried, just curious.