Why are the results different from ffprobe?

General Discussion
1

Audio and (Video?) file metadata is different with ffprobe.
Is this correct behaivior? or the bug?
I think ffprobe's results good.

The file (yt-dlp -f 251 -x --add-metadata https://youtu.be/IidyyM3giZw )
yt-dlp-output.opus
ffprobe result:

Metadata:
        encoder  : Lavf61.7.100
        title  : 【アナデン】Chaotic Recognizer (8bit Arrange)【Another Eden】
        date : 20210709
        purl : https://www.youtube.com/watch?v=IidyyM3giZw
        synopsis : アナザーエデン8bitアレンジCD，Another Eden 8bit Arrange CD，Track01
        comment : https://www.youtube.com/watch?v=IidyyM3giZw
        artist : Smiling Scene

mp3tag :

ARTIST : Smiling Scene
DESCRIPTION : https://www.youtube.com/watch?v=IidyyM3giZw
ENCODER  : Lavf61.7.100
LANGIJAGE : spa
PURL : https://www.youtube.com/watch?v=IidyyM3giZw
SYNOPSIS : アナザーエデン8bitアレンジCD，Another Eden 8bit Arrange CD，Track01
TITLE :【アナデン】Chaotic Recognizer (8bit Arrange)【Another Eden】
YEAR : 20210709
2

What exactly is different or not existing in Mp3tag?
Do you mean DESCRIPTION in Mp3tag which is comment in ffprobe?
Or the additional LANGIJAGE in Mp3tag?

3

Comment > Description
date > year
and, language
these.

I don't really know why language added.

4

These are just displaying names. Every software is using its own displaying names.
The technical field names actually used in the files metadata are often only short abbrevations.