Another reason for longer published times is the destination. If you are going into LHR, then you have to allow holding time and taxi time. If you are going into, say, Southampton, then times are shorter. This applies to many destinations.
Lastly, as you may well know, airlines now always quote the longest possible block time and a scheduled departure time that is well in advance of their slot time. This helps to get us on board and away - as well as any routing and weather delays - to ensure that the they can mark more on time arrivals.