Recorded audio in Audition, video on iPhone at same time. Won't sync

Hi folks,

I've recorded audio in Adobe Audition and video with the iPhone at the same time.  The idea is to replace the internal iPhone audio with the much better Audition audio.

When I upload both into Premiere, the audio file will not sync up.  I've used synchronize and hand adjusted the audio files to match up.    It starts out fine, matching peaks and valleys, but over time it gets off sync.  It seems like the Audition file is stretched out or the iPhone file is quicker.

I tried converting the iPhone video to constant framerate in Handbrake, and that doesn't fix the issue.

How can audio and video recorded at the exact same time have 2 different timings like this?