For anyone else who was confused by this, it seems to be a client-side audio compressor feature (not a server-side adjustment) labeled as "Stable Volume". On the web, it's toggleable via the player settings menu.
https://support.google.com/youtube/answer/14106294
I can't find exactly when it appeared but the earliest capture of the help article was from May 2024, so it is a relatively recent feature: https://web.archive.org/web/20240523021242/https://support.g...
I didn't realize this was a thing until just now, but I'm glad they added it because (now that I think about it) it's been awhile since I felt the need to adjust my system volume when a video was too quiet even at 100% player volume. It's a nice little enhancement.