The way the download system works is as follows:
1) isbndb and google books are queried based on title and author or optionally isbn if it is provided. In single metadata edit the user selects the best match, in auto the best match is guessed based on description length and a couple of other checks that I don't recall right now. neither of these plugins download ratings, but GB does download tags.
2) The isbn number from (1) is fed to the amazon plugin (and any other social metadata plugins) which downloads tags/rating/metadata. The rating is calculated as an average from all social plugins. tags are the union of tags from step 1 and all social plugins. comments are append.