Given the care that you took over collecting and checking your waypoints, I suspect that they are more accurate than anything else. The fact that repeat measurements came out within 1m of the originals indicates that there was little random error in the measurements, and in the absence of trees, cliffs and tall buildings nearby, there is no reason to expect systematic errors either. It is not unusual for the georeferencing of Bing imagery to be a few metres adrift (sometimes even the alignment of different Bing zoom levels can differ by that much). It is really quite unlikely that your carefully collected waypoints and repeated waypoints are over 8m out, so we have to conclude that the existing mapping is inaccurate. This is rather confirmed by the mapping showing a track that passes through the walls of the fort in two places.

Given the care that you took over collecting and checking your waypoints, I suspect that they are more accurate than anything else. The fact that repeat measurements came out within 1m of the originals indicates that there was little random error in the measurements, and in the absence of trees, cliffs and tall buildings nearby, there is no reason to expect systematic errors either. It is not unusual for the georeferencing of Bing imagery to be a few metres adrift (sometimes even the alignment of different Bing zoom levels can differ by that much). It is really quite unlikely that your carefully collected waypoints and repeated are over 8m out, so we have to conclude that the existing mapping is inaccurate. This is rather confirmed by the mapping showing a track that passes through the walls of the fort in two places.