If crossing the Atlantic Eastbound on a more Southerly track then normal, i.e. heading more for the French Atlantic Coast towards Brest, before turning northwards, I would often find myself off track towards the South.
On one occasion I was trying to understand the reason why, and the Skipper - who had been a Coastal Command Sunderland Captain in the days when they had to understand navigation techniques not normally of concern to mere pilots - said that he had experienced errors in this area, too, and was of the opinion that the charted Magnetic Variation in this area was incorrect, maybe the nicely curved isogonal lines in that area should show more of a kink. Maybe ?