So these counties never pruned their registration rolls for decades. A very common problem.
An Obama administration ruling demanded that Southern states (under Civil Right voting mandates from the 1960s) had to prune their voting registration rolls. When these states took the required Federal action to do this -- mailing a post card to the a voter at that address and requiring it not be returned as undeliverable -- then the Democrats started screaming about voter suppression. You can't win.