I expect the positional accuracy at maximum distortion is a closely guarded commercial secret, but it would not surprise me if it were to be plus or minus several metres. So the distortion is not constant throughout the mapping.
The map is made up from many images, which are put together and the surpluses discarded. But the images are not orthographic and there is distortion increasing from the centre of the image. The aerial images are taken individually from a great height. To try and explain the apparent discrepancy between map and drawing, one should go and find out the process of creating the map.