I'm not suggesting, Ken, a DIY recalibration of the RNAV approach, only monitoring whether the ice cap has shifted since the approach was created. This wouldn't require specialist equipment and I'm surprised it isn't done.
Ok465, I agree 50' is not an unreasonable RNPerror; that's why RNAV minima is higher than was used here! ...but ice cap shift would be on top of any RNP error. Its entirely your speculation which was responsible for the error observed by the crew. If we accept your view though, the crucial 50' could have been in a different direction after the crew had cool-headedly factored it in. Which merely emphasises that for all the undoubted skill of the crew, luck played a good part in this successful outcome.