The big problem with said hunks, is that they lose mass (e.g., abrasion, desorption..), AND they gain mass (e.g., dust and fingerprints, adsorption). This happens on the order of micrograms per year, IIRC. All the international standards have been drifting apart from each other independently, including the official standard kg itself (of course, since it was defined as the standard, we wouldn't say that it was varying, but, y'know).
Tim