How does Microsoft Teams decide which users will *remain* onscreen during a video call?

I have the same question, now that the 3x3 layout (i.e. 9 participants) has been rolled out. I love that I can see 9 people now, but in a recent meeting where there were approximately 120 people in the meeting, the layout didn't change every time a new person spoke. Anyone who wasn't speaking was muted, yet a few people who barely spoke, or didn't speak at all, remained on the screen for almost the entire hour-long meeting. I was told by several people that they could see me for most of the meeting, well before I spoke and afterward. I only unmuted myself briefly, for a 30 second comment, then muted myself again. Yet even after 9 additional people spoke, I still remained on people's screens. On my own screen there were a few people who didn't speak at all, nor did they have any "activity" going on around them, yet they remained on the screen. Why is that?