Facetime HD camera creates frame rate mismatch

When I use the Mac’s built-in camera to record video in iMovie, it gives me the option to set the aspect ratio but nothing else. When I create a new project, it gives me an option to set both the aspect ratio and the frame rate.

However, the built-in camera seems to record video at 20 fps but the frame rate popup in the New Project window offers only 30, 25 or 24 fps. This results in an unavoidable frame rate mismatch.

When I hover over the tiny fps number at the lower left of the clip in the Clip Viewer, the little yellow popup says that the “frame rate (20 fps) is not a multiple of the current projhect (30 fps).”

I know that when exporting the frame rates will be automatically converted to match but I was wondering if this frame rate mismatch exists for a reason or if there is anything I can do to avoid this?