It's third lightest---there's an outlier that's way lower. But, again just from eyeballing, it looks like there's two clusters, one that has tracked with the I

It's third lightest---there's an outlier that's way lower. But, again just from eyeballing, it looks like there's two clusters, one that has tracked with the IPK and one that is now heavier than the IPK. Based only on that, it strikes me as more likely that some of the national copies have gained mass(/weight), due to storage conditions or whatever, while others have kept their same weight. Of course this "doesn't matter" in that whatever the mechanism, it's still a problem for the science!