Not on HN sometimes some gems of comments are made by experts

Sometimes: when you least expect them. Not expecting them is a good strategy here.

I think the real question is: where is the evidence that scientists find it mysterious? That doesn't need any sort of explanation, just a reference for a claim made by a commenter. As with the other commenter, I've heard about the mass divergence before, but never that anyone found it surprising.

As has even been discussed on HN over the years (!), the cause is unknown.* This is one of the motivations to replace the official kilogram with a definition based on measurable natural quantities, as had been done with the second and the meter. It took until 2018 to do so: https://www.nist.gov/si-redefinition/kilogram-introduction

* 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!

I'm neither seeing the cause being mentioned as unknown in that link, nor would the cause being unknown imply anything being mysterious?

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?

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!

I don't disagree with you, but on the other hand: if people claim something in a discussion thread we need to be able to ask them to back those claims up. Otherwise anyone can just assert anything.

Sure, the explanation might not fit in the margins here, but then a link to a source that does attempt to explain it would be fine too.

One reason why questions like this are important is because it gives the audience around the asker an opportunity to learn something new. Isn't it important to be able to express curiosity, especially for the benefit of others?