Creating a video with MediaExtractor and MediaMuxer produces a video with shorter duration than the original one

For a testing purpose I am creating a new video from existing one by using MediaExtractor and MediaMuxer. I expect the new video to be exactly the same duration as the original one but it is not the case. The new video duration is slightly shorter than the original one.

fun test(firstVideo: FileDescriptor,  outputFileAbsolutePathUri: String) {
    val extractor = MediaExtractor().apply {
        this.setDataSource(firstVideo)
    }

    val muxer = MediaMuxer(outputFileAbsolutePathUri, MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4)
    try {
        val MAX_SAMPLE_SIZE = 20 * 1024 * 1024
        val bufferSize: Int = MAX_SAMPLE_SIZE
        val dstBuf: ByteBuffer = ByteBuffer.allocate(bufferSize)
        val bufferInfo = MediaCodec.BufferInfo()
        val indexMap = setMuxerTracks(extractor, muxer)
        muxer.start()
        muxDataFromExtractor(muxer, extractor, indexMap, dstBuf, bufferInfo)
        muxer.stop()
    } finally {
        extractor.release()
        muxer.release()
    }
}
               

private fun setMuxerTracks(extractor: MediaExtractor, muxer: MediaMuxer): Map<Int, Int> {
    val indexMap = HashMap<Int, Int>(extractor.trackCount)
    for (i in 0 until extractor.trackCount) {
        extractor.selectTrack(i)
        val format: MediaFormat = extractor.getTrackFormat(i)
        val dstIndex = muxer.addTrack(format)
        indexMap[i] = dstIndex
    }
    return indexMap
}

private fun muxDataFromExtractor(muxer: MediaMuxer,
                                extractor: MediaExtractor,
                                trackIndexMap: Map<Int, Int>,
                                dstBuf: ByteBuffer,
                                bufferInfo: MediaCodec.BufferInfo) {
    var sawEOS = false
    val initialPresentationTimeUs = bufferInfo.presentationTimeUs
    while (!sawEOS) {
        bufferInfo.offset = 0
        bufferInfo.size = extractor.readSampleData(dstBuf, 0)
        if (bufferInfo.size < 0) {
            sawEOS = true
            bufferInfo.size = 0
        } else {
            bufferInfo.presentationTimeUs = initialPresentationTimeUs + extractor.sampleTime
            bufferInfo.flags = extractor.sampleFlags
            val trackIndex = extractor.sampleTrackIndex
            muxer.writeSampleData(trackIndexMap[trackIndex]!!, dstBuf, bufferInfo)
            extractor.advance()
        }
    }
}

Just for the sake of comparison the original video duration was 3366666 microsec and the created video duration was 3366366 microseconds. The video length is retrieved from MediaFormat (MediaFormat.KEY_DURATION)

It appears that it depends on the way the source video was created: When I used ffprobe to get source video metadata I got the following:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'source.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.76.100
  Duration: 00:00:03.37, start: 0.000000, bitrate: 123 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 400x136, 118 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
    Metadata:
      handler_name    : Core Media Video

The result video metadata was:

 Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'result.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2022-01-06T12:01:21.000000Z
    com.android.version: 11
  Duration: 00:00:03.37, start: 0.000000, bitrate: 126 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 400x136, 118 kb/s, SAR 1:1 DAR 50:17, 30 fps, 30 tbr, 90k tbn, 60 tbc (default)
    Metadata:
      creation_time   : 2022-01-06T12:01:21.000000Z
      handler_name    : VideoHandle  

When I use the video that was created by using MediaExtractor and MediaMuxer as a source the video duration is the same (within 1 microsecond threshold)