This issue has been identified as a product defect and is being tracked as Defect ID 230154
Steps to Reproduce:
1. Complete a successful sync in Contact then do not modify anything in ARS properties contact and Run sync. It picks up all the contacts in the scope to sync again.
2. Complete a successful sync in Contact then please remove update rule attribute values from couple of contacts in ARS properties and Run sync. It picks up again all the contacts in the scope to sync again.
The same behavior is not observed for the User and group objects. The only difference is in the amount of users that are being processed.