Until about 50 years ago the historic (or traditional) counties of Britain were universally used as the standard popular geographical reference frame of Britain. The location of every village, town and city of Britain could be simply described by reference to one of these counties. We literally knew where we were and local government areas were closely based on these counties.

Ongoing local government re-organisations have resulted in the creation of administrative areas which often now bear little resemblance to the original county boundaries. Many of these administrative area names have been adopted as county names. 

To add to the confusion the Royal Mail maintained county names on the postcode address file until December 2000 but did not update in line with local government re-organisations so we effectively ended up with a third type of county - postal.

That's why a postcode can lead you to 3 different "counties" - For example for many "BA" postcodes the postal county (no longer used) is "Avon", the traditional county is "Somerset", and the administrative name is "Bath & North East Somerset".