Sc.doc.Materials (Materials table) - I don't get it

I have a file that has perhaps 25 materials in it - listed in the material tab. If I call sc.doc.Materials to get the material table it is 440 indices long… How is that possible? There are many objects in the file that share the same material (I see the same name), but each one has a different material index!

(yes, I have purged unused materials)