Why do measurements of the gravitational constant vary so much?

(Phys.org)--Newton's gravitational constant, G, has been measured about a dozen times over the last 40 years, but the results have varied by much more than would be expected due to random and systematic errors. Now scientists have found that the measured G values oscillate over time like a sine wave with a period of 5.9 years. It's not G itself that is varying by this much, they propose, but more likely something else is affecting the measurements.