Why do people who live in Petts Wood say that it is in Kent when in fact it is in Greater London?

Petts Wood is in Kent for the same reason that Orpington is in Kent and Bromley is in Kent, they are all in the London Borough of Bromley in the County of Kent - these areas do not have a London post code. Has anybody out there ever written to someone at 92 Blah Blah Road, Greater London? If they had would they really expect the letter to be correctly delivered to the addressee? Come on people think!