I made another export using the H.264 Blu-ray preset, and just imported the
resulting m4v and wav files into my Encore project: same results.
Video duration = 01:20:04:01
Audio duration = 01:20:07:01
Source file used in the Premiere sequence is 01:20:07:01. Something is
happening in the export process that's causing the video file to come up
shorter. A frame rate conversion could possibly do that, but it would do it
equally to both video and audio.