[FFmpeg-user] Extra video frames were added when doing the resolution transform

[Du Dengke]

Hi all
When I run the following command
    ffmpeg -i test.mp4 -vf scale=640x360 test2.mp4
where test.mp4 located at:
https://cdnproduce.yunshicloud.com/43E0AE6C22824961/FABU_YUNSHI/EF5649D3ADB5458EB06F15D2CD719C61/4F32E11AFFC44612BACC4246FCE0A9DF.mp4


And run the ffprobe on test.mp4 and test2.mp4 respectively:
test.mp4
    ffprobe -i test.mp4 -show_entries stream=nb_frames
video stream show:
    [STREAM]
    nb_frames=5822
    [/STREAM]
test2.mp4
     ffprobe -i test2.mp4 -show_entries stream=nb_frames
video stream show:
    [STREAM]
    nb_frames=5854
    [/STREAM]

32 frames was added after the transform, and I saw some static picture
within the first second.
My question:
    Why the 32 frames was added, does this video trigger some mechanism?
And why the static picture was added at the first?

//dengke