I hope the attachment below helps a bit. It covers just a small number of the very many permutations!

There are 4 basic setups, and assume an extremely low res camera model, so you can see the fov of an individual pixel.

Sketching this really helped because it shows that the reconstruction uncertainty value (as i thought i understood it - maybe i was wrong!) is not completely reliable.

#1. Cameras close together. almost parallel cameras looking at a point on a ground plane more or less dead ahead. XY uncertainty is 1.38 and Z uncertainty is 12.706, so the overall reconstruction uncertainty is 9.207.

#2. Cameras far apart, angled to look at the same point as in #1 in the center of the camera fov. XY uncertainty is slightly higher than #1, but Z uncertainty is much better, and so is overall reconstruction uncertainty.

#3. Cameras far apart, same distance as #2, looking parallel to each other. The same point as in #1 and #2 is now in a small area of overlap at the edge of the cameras fov, which has barely any effect on uncertainties and the reconstruction uncertainty is just very slightly higher.

#4 Cameras back close together again, same distance as in #1, but with the point of interest near the edge of the fov of both cameras. The uncertainty here is actually lower than in #1 for identical camera positions and orientations.

** This may be misleading though, because the XY uncertainty is greater than #1, and so is the Z uncertainty, so should be more uncertain overall, but it is the ratio between them that I thought was reported in photoscan.**#5 Identical to #4 except taking the 'Z' uncertainty as the maximum distance rather than perpendicular to ground, I'm not sure which is actually the correct one in this case, but still shows more or less the same result.

Now I am much more confused than I was to start with