Why did my transatlantic flight meander off the great circle?
Most flights across the Atlantic don't fly direct great circle routes; they use the North Atlantic Track system, which is an array of "lanes" you might say, usually 60Nm apart, like one of those slot car racing places with 6 or 8 lanes of cars.  Just about everybody that can make the trip non-stop will be in the tracks at various altitudes. Off each coast are an array of fixed entry point fixes called Oceanic Entry Points as jumping on or jumping off points. 

The north/south position and the precise routing of the tracks is set up each day, with the focus on eastbound traffic in the morning and westbound in the evening, and are moved up and down the take advantage of winds or weather, especially jet streams.  

Jet streams flow along air mass boundaries at the top of the troposphere and change constantly as air masses move.  The tracks are set up to try to exploit or avoid them depending on whether they are favourable or not (as well as other weather systems).