OSX Incorrect Date & Time {while connected to the internet}

When a Mac is hooked up to the internet why would it lose track of the time. I noticed it was off by a few hours but was set to the right time zone and knew my exact location in system preferences/date&time. I watched it for a few days to see if it would just snap back with its connection to the internet, letting the computer know that it may have a mind of its own but it is not the time keeper... Never happened on its own. I had to unlock system preferences/date&time to edit and it snapped back. No edit needed, it did it on its own as I expected it to earlier. What creates this disconnect? How can it be avoided?