Audio waveform changes while adjusting video/image length?

Hello!

Hoping someone can explain what iMovie is doing here - I've looked all over for an answer to no avail...

In an iMovie project with

- a single png file - dragged to always be shown during an audio file - my song

- an audio file of my song

As the length of the png image is varied, the end point of the image causes the audio track to seemingly change/dip in response.

The attached image shows the behavior.

Audio always plays normally, although the playhead 'hops' over the 'dip' in the waveform of the (green audio file).

So, it isn't a 'problem' but since the audio waveform changes in response to adjusting the visual, I assume iMovie is trying to indicate something with this behavior?

Thanks for any help! 🙂