Based on the USA guidelines most roads outside of urban areas should be classified as unclassified. Most roads in this area were originally imported as residential roads, well before that guideline was published. 

The second reason is the definition of the highway=residential tag states the roads are in urban areas. 

Based on that and other reading of the forums it appears that most roads in this county were incorrectly imported as residential and should be classified as unclassified. Only roads in urban areas should be classified as residential. 