Why does autoclassify leave some files behind?

I have a nicely nested, reasonably complicated group structure. I have just imported all my 2013 pdfs and they are sitting in the inbox. When I highlight them and click autoclassify, the log reports

about 10% skipped
about 30% not classified
about 25% added
about 25% classified

Why is there a diversity of outcomes? Why are some readable PDFs not classified? What is the difference between added and classified?

I was hoping by this time (5+ year-user) I wouldn’t have to classify and drag hundreds of docs on a periodic basis.

Thanks for your help,