Back in the old days, even top 40 stations sounded different from market to market. Sure they all played the big nationwide hits (Motown, Beatles etc.) but there were huge regional differences.
The old AM top 40 stations in Nashville, WMAK and WLAC and WKDA back in the 60s played far more soul records than other parts of the country. This was even more true in Memphis on WHBQ and WMPS and the New Orleans rock stations WNOE and WTIX were almost 60 percent R & B.
WLS and WCFL in Chicago only played the really big soul records put played lots of British Invasion you never heard in the South.
KHJ in Los Angeles played lots of sunshine type pop that was not heard anywhere outside of Southern California.
KFRC played lots of psychedelic pop heard only in San Francisco.
WABC in New York pretty much stuck to the top 10, but in the 1970s played a whole lot more disco than stations in other cities.
Today, a top 40 station in Seattle will sound just like the one in San Diego or Miami or Boston.