I had the RAC out repairing my car a little while back and he said that the ECU 'learns' your driving style and adjusts itself accordingly.
I have noticed with my car (2004 1.4 Petrol Ford Fiesta Zetec) that sometimes the acceleration seems more urgent than it does at other times. In recent weeks I've been trying to save fuel and driving economically, and I've noticed the acceleration was a tad sluggish when I put my foot down.
However yesterday I took the car out for a quick blast down the motorway, by the time I got back I found that it seemed to be accelerating a lot faster and just seemed to have more power in general. Could the ECU have switched modes from economic to performance somehow? Or is it just all in my imagination...