"Why limit the sources?"
Well, the metadata source plugin is rubbish at finding the right book unless the ISBN is populated - and even then it often goes and finds a different book. With this example, you managed to "find" the right book by massaging the search data - sometimes you have to try multiple versions before finding a book, and sometimes it comes back with the wrong book and it isn't immediately apparent. When an ID exists, the plugin SHOULD use it - otherwise it's just screwing up your database.
Note, the Goodreads sync plugin is fantastic at finding the right book - you can quickly see all the relevant options for each book and pick the right one.
Looks like I'll have to investigate modifying the Goodreads metadata plugin
Dang it, I hate Python!