PanAm 103 was a delayed flight from LHR. It would therefore have filed a flight -plan for a 'random track' across the Atlantic. The Organised Track System - OTS would then be eastbound and PA 103 may well have filed a more northerly track ( not so unusual) to avoid conflict later on and eventually climb to its' optimum altitude with few restrictions, or the PanAm planners may have tried to get a routeing in the least-effective headwind.
The model used by flight planners depends on the airline, but usually takes into consideration, such variables as headwind/tailwind and military/political constaints. The fixed factors are organised tracks ( not only the Atlantic), fixed Mach number technique -used for spacing and some other factors such as high safety altitudes and oxygen requirements. Despite having selected the fastest flight time (and therefore the least fuel required/ most economical) this may not be the route used - maybe a hurricane/tropical cyclone/typhoon on the way. Also the track requested may not be available/ or altitude/ or speed, due to congestion. No-one said it would be easy!!