Here's what I think:
Often songs over the radio are not played at the correct tempo. What I mean is this. Radio stations try to fit in as much music as possible per hour and still have time for commercials.
So, if they can speed up the songs by a few seconds -they save time and can play more music and advertisements.
The equipment and program that is used to speed up the tracks is supposed to work without distorting the music other than tempo -but, sometimes it does. This is really noticible on some songs on a couple radio stations around here.
This equipment is also partially responsible for the 4-7 second gap between the music (in the station) played in real tempo and the music heard by the listener (compressed audio.)
Not to mention that radio waves distort signal.