Erebus has been in exactly the same position for a very long time.
So has McMurdo Sound.
That is/was the problem.
He thought he had been briefed to fly down the Sound, not into the Bay.
So had the other crew at the same briefing.
That's why they (the surviving crew of the same briefing) were surprised to see the 26 mile cross-track 'error' on the INS when they found where they were in a less malign form of 'VMC', on a similarly briefed flight.
Look at the videos, please.