Over three decades i clocked up a great many trips from London to Bermuda (32N 64W approx 700nm SE of JFK) during that time flying on 747 classics, DC10s, tristars and 777s I have flown over the Azores and Iceland en route and pretty much everywhere in between including coasting out over Lands End and not being over land again until 50ft over the lights at BDA .
Atlantic weather can certainly cause a huge variation in tracks and as has been said thats before Shanwick have to sort out everyone wanting the same few tracks and flight levels -of course there was always Concorde to JFK which was literally above all of that.
Also had some extra ordinary variations in block times too including close to ten hours westbound once (1011-500) and 5 hours 5 mins back a couple of days later