Did you click on the other kilogram links (or other metric links which are also interesting?)
A useful paragraph is the following: "The trend during the past century had been for most of BIPM's official copies to gain mass relative to the IPK, although by somewhat different amounts, averaging around 50 micrograms (millionths of a gram) over 100 years. But an alternative explanation is that the IPK was losing mass relative to its copies. Even more likely, it was a combination of both."
Those pages are for the lay audience but you can do your own web search (probably even an algolia search of HN) to know more.
In general, it's often easy to come up with an explanation of some new phenomenon, but to answer the question you still have to do science...which often simply confirms intuition but not always. My comment was less on the reason and more on the difficulty of doing experiments on what was literally the kg, no matter how it fluctuated.
My theory: People are treating them with different degrees of reverence. They are careful to polish anything off the IPK, causing it to lose weight gradually through abrasion. The others gain wait through random adhesions, perhaps oils picked up by gloves used to handle them.
I'm just some random guy on the internet, I know. It's fun to have theories!
A useful paragraph is the following: "The trend during the past century had been for most of BIPM's official copies to gain mass relative to the IPK, although by somewhat different amounts, averaging around 50 micrograms (millionths of a gram) over 100 years. But an alternative explanation is that the IPK was losing mass relative to its copies. Even more likely, it was a combination of both."
Those pages are for the lay audience but you can do your own web search (probably even an algolia search of HN) to know more.
In general, it's often easy to come up with an explanation of some new phenomenon, but to answer the question you still have to do science...which often simply confirms intuition but not always. My comment was less on the reason and more on the difficulty of doing experiments on what was literally the kg, no matter how it fluctuated.