>“We’ve moved the lab 14 times around the sun,” says Patla. He’s not trying to be cute; it turns out that Earth moves through some distinctive regions in space. Because Earth’s orbit isn’t a perfect circle—its distance from the sun varies—the clock moved through varying gravitational fields.

Michael from Vsauce has a nice graphic showing that https://youtu.be/IJhgZBn-LHg?t=1016