When syncing to a CSV file (or other source) there should be value(s) to key off of and update existing records. It sounds like the referenced record is already updated providing that the key value was found and the default settings are in place. (which is to add the record if nothing is found). The sync logs should have more details. You can view them by clicking on the sync package in the web console. There is also the community article below that will enable debug logging for the synchronizer.