Audio waveform shows max audio despite actual level varying

I've found the answer to this problem, so this entry is to share it in case others have the same situation.

One of the reasons I use iMovie is to raise or lower the audio in different portions of the video so it is uniform across the whole thing. Yesterday (with MacBook Air M1, macOS 15.0, iMovie 10.4.2) after loading the video, I noticed that the audio levels for the whole video were either right up to line or yellow peaks. There were no dips below the line, even though I knew there were periods with no audio at all.

Eventually, I checked Reduce background noise. In a few seconds, the audio waveform became normal. I found that it didn't matter what percentage I chose, 1% worked the same as 100%. By the way, I could not hear any background noise myself.

I tried an older video, and did not have this problem. Whether the problem is something in the Theater where I was recoding (didn't have this problem before) or in iMovie, I don't know.