-1. If a detail, the slight deviation of the nose, is present in the negative image it is obviously codified also in the positive image. Perhaps our eyes-brain system is not able to detect the detail in question because the inverted colors introduce some difficulties in the subjective interpretation.
I want not to discuss here the evidence of the broken nose (and then slightly deviated) on the Shroud image because this fact has been already showed by medical forensic experts.
-2. Minting error or damage to the coin? It was the first problem I considered but higher magnification of the detail of nose eliminates this hypothesis (see . . . [image at right, click on it for larger 692 by 768 pixel version]. In addition I have also found the photos of a Justinian II semissis, a Justinian II solidus and a Michael III solidus showing more or less a deviation of the nose (with the same curvature) that confirm the detail perceived by different Byzantine sculptors.
-3. At the time of the Byzantine emperor the Shroud body image was certainly more evident than now because more contrasted from the background that was brighter. It was therefore easier than now for an observer to detect details like non-symmetric hair.