Flight duration changes

I'm booked on VS077 in December and the other day received a notice that the flight times have changed. Does anyone know why a flight in July should take an hour less than the same flight in December?
Spill wrote:It is likely they are expecting more delays in December so therefore they extend the duration so as not to look bad.
This happens all the time.
There are short haul routes in the UK that have 50% addoitional time from the shortest to longest depending on the time of day. and obviously the distance remains the same.