From Harold Schutzman:
Why is the top of the hour time tone off by 8 seconds?
WCBS now broadcasts in high definition. It takes 8 seconds for HD to encode and then decode at a HD receiver. Consequently, the regular analog signal must be delayed 8 seconds in order for the broadcasts to synch. Also, it is not as simple as starting the time tone 8 seconds earlier to make it hit the airwave "on time." The issue is much more complex. However, WCBS is working to find a solution as HD radio becomes more mainstream.
For more information on HD radio, visit hdradio