A narrower follow-up, since the general answer is now clear:

How would you tell the difference between that and the alternative explanation?

Closing the loop on my own question.

Closing this out. What actually changed my mind was the point about waiting for a trend instead of reading a single measurement, which is embarrassing in hindsight and worth saying anyway.

kate.chem said:

One habit that pays for itself: post the method alongside the number.

Agreed on the substance. I would put less weight on the timescale, because two months of anything is not enough to distinguish a trend from a wobble.

kate.chem said:

One habit that pays for itself: post the method alongside the number.

Genuinely useful, thank you. I had the facts and not the framework. Adding it to my notes with a link back to this thread.

Clinical perspective, offered as context rather than as advice. If two explanations both fit, the useful question is which one predicts something the other does not. That is answerable; arguing about which sounds more plausible is not.

Correct me if the detail matters more than I have assumed.