[_JJ_]

Yes indeed, MP3 files include an extra sliver of time at the beginning of the file. I don't know if it's some kind of "header" in the code, or a metadata chunk, or what, but the result is as you've described.

This is why I always insist that any mp3 audio sent to me for work must have at least one click (or any sound) out in front of the music that can be used as a sync point when importing / exporting / reimporting.

I'd be curious to know the technical reason if anybody here knows. WAV and AIFF files don't appear to suffer from this timing anomaly.