Drafts Folder in Mail.app contains messages which have been sent
Gmail accounts are specifically subject to this issue.

In this Apple Support Communities post, there is a good discussion of why this occurs and how to reduce or stop it. To quote a particular response:


  Mail creates a temporary draft each time you compose a message. If you
  send the message while the IMAP account syncs, the draft doesn't get
  deleted and gets stored in gmail's draft folder.
  
  If you want to stop this, uncheck to store drafts on the server. It will put the draft
  folder back under the gmail folder and create a local draft folder for
  gmail. If you need to complete a draft on another device, simply move
  it to the IMAP draft folder.


I can verify from experience that this issue occurs in Mountain Lion as well.