The form shows a success message but subscribers are not added to my list(s)?

I am using the following settings to process 1,000 subscribers on my blog to my list.

Settings

I’ve ran the manual sync twice and although the progress box shows 100% they still don’t appear on my Mailchimp subscribers list. Perhaps I’m misunderstanding how the feature works… Do the subscribers go into a ‘purgatory’ stage in mailchimp, who then sends the opt-in form to the email and only then do they get added?

Maybe I’m missing something?