I was asked today why the percentage would jump so quickly and then go down. I had thought the percentage was some kind of completed status, but couldn’t determine to what, so had to admit I didn’t know. I have googled and searched and looked at the code for a potential class name hint to no avail.
Files loaded, pixels rendered (related LCP?), or what? What is the percentage a percentage of?
The percentage shows the visual change on the webpage. After FCP there usually is huge shift in the webpage when all the elements of the page are loaded, hence you see the jump. You should be able to see the same behaviour in the real browser as well.
And regarding fluctuations it could be because of dynamic content loading, asynchronous requests, or rendering complexities.
The percentage is relative to the end state (last frame) so it can go up and down depending on how the contents change. In the example above, it looks like the text at the bottom shifted slightly out of the viewport, reducing the amount of “black” area in the viewport. Presumably more frames after that add more black content and the percentage jumps back up.