DropBox and Time Machine

By default, TM backs up everything that is stored on your Mac, so unless you have specifically "excluded" DropBox from being backed up, TM is backing up DropBox and the DropBox files stored on your Mac.

Time Machine does not backup files that are not on your Mac. If you have files stored on Dropbox that do not have a local copy on your Mac they cannot be backed up by Time Machine.