ADM BWF files are not automatically trimmed and padded to frame boundaries on export. If your master file was created with a free record start and end (rather than using the in/out points) it may start or end in-between exact frame boundaries.  The start and end times shown in the renderer are shown rounded to the nearest frame however, the export will be the exact length of the file – resulting in a slight difference in duration being shown. If you want your export to start and end on a frame boundary you will need to specify the in/out points when exporting.  Please note that IAB.mxf files are automatically padded and trimmed to frame boundaries on export.