BB
I'm not sure if you're talking about accuracy of the kit in your aircraft or that on the ground? Rest assured, however, that the dimensions of the final approach segement (the area considered for obstacles when determining the approach minima) is considerably larger for a non-precision approach (like NDB) than that for a precision approach like ILS... particularly at the FAF end of the segment (where you might say accuracy is most relevant). This probably takes into account the 10 degree difference you are experiencing and more.
Another factor that you might want to consider is that the FAT has to be aligned through the NDB (as that is the only means of obtaining course guidance) and yet pass within a given distance from the runway threshold (IIRC within 0.5nms of THR?). So the FAT for such an approach may not be aligned with centreline, deliberately.
Remember the aim of a non-precision approach is that you are able to see the airfield by the time reach the FAF... whereas on a precision approach you should be able to see the runway and have a much better chance (I can't remember the percentages, but they are quoted) of landing from the approach?
Hope that makes sense?