I had the same thing happen after my batt.(BMW) died of old age.My mech. said that the computer has a short term memory that remembers your driving habits and adjusts for optimum mileage(in theory anyway).If you take you car out and drive it hard(i.e. run up the revs.) for a few miles it reeducates the memory.It must be true,as I've it a couple of times since.