Ok I now see that every file has the same metadata (song name, album name, etc). Exactly as you said it was.
Now the question is why. Do you have a program to display tags that is not MC? It would be nice to see the tags of one or two of your files before you do an import and then see them after. I suspect that the files in question are tagged with what MC shows.
KID3 is a tag editor I have used in the past. MediaInfo would probably be even better, as it is not an editor; it just shows values. Even VLC would work. VLC will show you Album, Artist, etc when you press Command-i on the current file.
I really don't know what could be going on if Tag On Import isn't set up. MC gets the values from the file metadata. That's why I'm asking you to examine it.
Best of luck,
Brian.