Record Change Detection
Changed records in Salesforce are identified based on the SystemModStamp field. This is updated for any change to a record. This can include simply viewing a record as this updates the LastViewedDate field. Therefore the incremental download may identify records as changed even if no material change has occurred.
Also note that some changes may update a lot of records unexpectedly. For example changing a picklist value description will trigger a change to every record that uses that picklist value. This may trigger large downloads for objects using the incremental method.