Metadata editing

For the most part I've noticed that MP3Tag is successful at editing tag information from music files that come my way. A number of my music files, however, contain original tag info that I am surprised to find is still there! And that I am unable to remove with MP3Tag. I have no idea why this is and how to go about removing these artifacts, eg. album covers. Is this because another (presumably more robust) type of tag system was used in first-assigning the metadata? Thanks!

Which program still shows you the unwanted metadata?

And check Tools>Options>Tags
for the kind of tags you read and write. E.g. if you read but do not delete or write APE tags then you will always see their contents as APE overrules ID3