SyncApps always tries to sync only modified data between your software applications. This method is a best practice rather than trying to sync all data each time you run your SyncApps.
Let's face it sometimes the APIs of different software apps go down so you might not have your marketing Lists populated in time or you need to true up your CRM to your Marketing Automation solution.
If all of your data needs to be re-synced, at any time, please click on the "Reset Sync Process" button and during the next sync process all your data between your software applications will sync.
*Please note - you will not lose your previous settings in your Sync Profile.