Not sure if this is one for pedants/anoraks corner, but it aroused my curiosity a bit.

I just got an alert from MyFlights to tell me that my LIS-LHR flight in August would be arriving at 10.45am rather than 10.50am. No other changes - same equipment, same departure time etc...

So my question, on what basis does BA determine that the flight time is going to be 5 minutes shorter? Too early to decide based on loads I'd have thought, likewise can't predict weather that far ahead, so are they perhaps being told by LHR that the slot has moved and they will just have to fly/taxi a bit quicker and presumably not have such an optimum fuel burn?