* it’s not like there aren’t good theories but you can’t experiment directly on “the” kg because you can’t risk changing it!

Like if you find that your car has broken down after 200k miles, you might not be able to determine the cause, but it wouldn't exactly be some kind of mysterious physical phenomenon that would puzzle scientists. Obviously, something wore out. Why/how is this any different?

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.

I'm just some random guy on the internet, I know. It's fun to have theories!