clockspeed and NTP

Hi all,
I have a small question.
Dan says in his install doc.
Find an NTP server that you trust. Some public NTP servers are
......, but you
will probably obtain more accurate results from a local server.
Why, aren't NTP servers throughout the world suposed to show the same time?
Why does he says try to get an local server? 
Thanks a lot in advance and bye.
-Payal