BOAC
I believe I have addressed in detail several times why, retrospectively, AAIB and Boeing ascribed a useful (to analysis) accuracy to the position at waypoint change derived from the STANS data (working back from the data at impact correlated with the recoded data to Corran at waypoint change).
The crucial thing is that the pilots did not have the benefit of this retrospective and would not, indeed, have had confidence in that position after a sea crossing and whilst still over sea - such that had some other local point reference contradicted the STANS, say by 1/2 mile or so, they would have preferred to believe the local reference.