A county planner is trying to add a scanned historical map i…

A county planner is trying to add a scanned historical map in a GIS.  The map was georeferenced using an affine transformation and 4 control points.  The RMS errors were all quite low.  To the county planner’s dismay some of the map’s roads and landmarks do not all match with the modern map.  Which of the following is the best explanation for this?