My macbook pro clock is always three minutes fast?

I have an early 2015 macbook pro running high sierra, 10.13.1.

In date and time preferences, the "set date and time automatically" box is checked.

However, comparing the time to my iphone time and googling "what time is it exactly", I can see my clock is three minutes too fast.

If I uncheck the box, then recheck it and save, the clock adjusts to the correct time. But then gradually over the next few hours it becomes a few minutes too fast again.

If I uncheck the box and choose to adjust the time myself, it still becomes too fast.

Any suggestions? I feel like it's not worth resetting my mac or going all the way to the apple store over just a few minutes off, but it's driving me crazy!