The short answer is that the "Attributed" tag means we have a strong suspicion that the quote is wrongly attributed and is not really the work of the quoted author.
So why is the quote still there? One of two reasons:
1. We haven't determined the true attribution yet, but we don't want the quotation to be missing if someone searches for it.
2. People commonly attribute the quotation to the supposed author, so we include it under their name where people will be looking for it. In these cases, there is usually a note indicating the true author. See for example the following quotation commonly attributed to Voltaire:
Ideally we would only have quotations in the second category, but we have a limited amount of time for tracking down quotation sources.
Incidentally, you would be pleased to know that there is also a "verified" flag in our database, which will indicate quotations with a verified attribution. This will become visible when we update the site in a month or two.