30fps from iPhone becomes 29.97fps in iMovie - why?

I don’t know the reason for the difference.

If you import a 30 fps video clip into a newly created project, that will set the project at 30 fps and the clips in it will play at 30 fps. If that’s what you are doing and it still doesn’t work, then try deleting preferences by opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Navigate back to your old library. Now see if the clips sync.

You might also be able to make a tiny adjustment in the speed of the video clip, or slow down the audio clip a smidge. Try control-clicking on the video clip and selecting Show Speed Editor from the drop down menu. That will cause a speed adjustment handle to appear in the upper right hand corner of the clip. Expand out your time line with the slider located above the timeline to the right. Then slide the speed adjustment handle to the left to speed up the clip, and to the right to slow it down. Hopefully the adjustment will be precise enough to bring the clips in sync. You can do the same procedure with the audio clip.

— Rich

Apparently iMovie converts 30 fps to 29.97, per your findings.

I recorded a video on my iPhone at 30 fps setting. QuickTimePlayer movie inspector showed it at 29.98. When I imported it into an newly created iMovie project, it showed at 30 fps, or at least the frame counter clicked from 00:29 frames to 01:00 (1 minute) on the next click. So that indicated 30 frames in one minute. Don't know how accurate the frame counter is, as we are dealing with very tiny rate differences. So can't be sure whether iMovie may have converted the clip to 29.97 fps on import. Your findings would suggest yes.

I exported the movie and QuickTimePlayer movie inspector showed it at 29.97 fps.

So, there may be some inaccuracies in the frame rate measurements. My 30 fps iPhone video exported at 29.97 fps. However, your real time measurement shows 29.97 fps, so that would be the proof, at least as to the export frame rate. I'm not sure whether iMovie made the conversion of the original clip upon import into iMovie or upon export. Your findings would indicate that it was converted on import. To answer your question, I don't know how to keep that from happening. Did you try using the Speed Editor to make an adjustment per my earlier suggestion? Either speed up the video or slow the audio? If so, did that work to bring the clips in sync?

Another thing you can try is to export your video clip and use Handbrake to convert the 29.97 fps rate to 30 fps, and then import the converted clip into your project. See if that makes a difference. You can get Handbrake here:

https://handbrake.fr/

-- Rich