It is indeed down to the forecast winds - somebody with better knowledge than myself can go into the finer detail but the westbound tracks for a particular day are produced the previous evening by one of the Shanwick controllers based on the forecast and the prefered routes of the airlines. There are usually six tracks and the bulk of the transatlantic traffic will fly along one of these, though some will fly on random tracks. The main idea is to avoid the srong upper winds (jetstreams) that blow from west to east. Conversely, eastbound tracks will try and utilise these winds which is why it takes less time to fly from Chicago to Manchester than the other way round.
It does come as a suprise that there can be so much of a difference - many years ago I flew on AA55 to Chicago and we entered oceanic airspace at 51N 15W. At work I quite frequently see the same flight entering oceanic airspace at 57N 10W. That is 360nm further north!