The map itself is just a large data base stored in a hard drive within the IFE system, the position of the aircfart on that map is derived from GPS, so any error on the map is in theory always going to be there.
have not noticed it myself, I think i'm usually in the flight deck when stansted is on the map!, will try and have a look for you.
Were you on a long haul 767 that day?, coz our short haul ones do not have a moving map display.