ntp is not "smart enough to realize you have three identical "server" lines in ntp.conf". Before the addition of the pool directive in 4.2.7 this was the recommended approach (as in recommended by prof. mills, harlan and the rest of the ntp reference implementation team). OSX is trying to be cute and thinks it knows more about ntp than the user.

Try editing /private/etc/ntp.conf and append:

  # use multiple ntp servers
  server us.pool.ntp.org iburst
  server us.pool.ntp.org iburst
  server us.pool.ntp.org iburst

Make sure you have an extra empty line above the comment. There is something goofy about the way osx includes the input from the graphical configuration and what is included in the /private/etc/ntp.conf In order to restart the service you need to disable/re-enable the automatically set time.