It turns out that the official reference weights for the “kilo” have been changing weight. Nobody knows why. In a few years, there will be a number of meetings to figure out a new way to define a kilo.
Classification and standards are closely related, let’s see how this works out. Reminds me of how Pluto was re-classified as no longer being a planet, not too long ago.
Peter Van Dijck 
That’s what happens when you base an SI unit on something which isn’t constant…
I had previously heard that it is the kilo mass itself which was decaying due to environmental stresses.
Or it could be a few hundred thousand radio-isotopes of the platinum or irdium which have decayed over time, resulting in a drop in mass.