Google’s John Mueller has advised that the PageSpeed Insights score can change without any changes being made to a website.
This information was provided on Twitter in response to an SEO who questioned Mueller about a fluctuation in PageSpeed score.
Mueller says these scores are not physical measurements and may change over time whether or not the website itself has been changed.
Ultimately, Mueller believes these tools should be used to find ways to improve a site for users.
Achieving a particular PageSpeed score should not be seen as a final goal.

I don’t know the details in this case, but generally speaking, these scores can change over time even without changes on your site, they’re not physical measurements. Use these tools to find ways to improve your site for users, don’t see them as the final goal.
— 🍌 John 🍌 (@JohnMu) February 5, 2019

That’s one explanation for the fluctuations in PageSpeed score the Twitter user is referring to.
Another explanation is a bug that affected PageSpeed Insights scores from Feb 1 to Feb 4.
PageSpeed Insights Bug
According to recent PageSpeed Insights release notes (Google doc link), there was a known bug in early February that led to lower scores.

“Between Friday Feb 1st and Monday Feb 4, the headless Chrome infrastructure used by PageSpeed Insights had a bug that reported uncompressed (post-gzip) sizes as if they were the compressed transfer sizes. This led to incorrect calculations of the performance metrics and ultimately a lower score.”

The bug was completely addressed as of Monday, February 4.
However, while mobile scores will return to their previous values, desktop scores are expected to remain lower.
As explained in the release notes, PageSpeed Insights now utilizes Lighthouse version 4.1.0 under the hood.

“Desktop scoring has been recalibrated to stay on par with the fast moving world of desktop browsing based on the most current data on global page speeds. You should expect desktop scores to be lower now than they had been.”

The upgrade to Lighthouse v4.1 will not affect mobile performance scoring.

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