No you are not doing anything wrong.
You will find these materials are loaded from a GDL file in your library.
This file will start with the name - MASTER_GDL_........
I can't tell you exactly what the name will be as I don't have the Cadimage add-ons loaded.
So all the time you have this in your library they will be added to your attributes every time the library is re-loaded.
i.e you can delete them but they will come back.
You will notice in the attribute manager that thier names are in itallic text.
The same can be done for line types and fills.
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