iTunes mp3 files usually have additional comment tags that store some technical information (loudness, ...)
Check "View > Extended Tags" in Mp3tag to see these comments.
These comments might confuse other programs and "hide" the main comment.
You should also know the Windows explorer ID3v2 tag support in general is not good, it has several flaws and it is not a reliable way to verify tags.
If you want to copy one field to another:
/t/967/1