Sure decay rates have been measured often, but with different sized samples, with differing levels of purity. When someone says the half-life of Cesium-137 is 30.1 years, that's because all the measurements over the years which have been taken averaged together to predict that rate.
This new experiment wasn't about the half life at all, but the variance between individual atoms decaying. They were paying very close attention to how random or not the release rate of particles was. When another mathematician repeated the experiment he found a small, but consistent change in the rate.
