Backbone synchronizes field changes based on the history of an issue. When an issue gets updated, it analyzes the history and synchronizes all changes which happened since the last synchronization.
However, there are some field changes which are not recorded in the history, e.g. adding/removing a watcher or a vote, scripted fields (by Scriptrunner), etc.
Backbone currently synchronizes these fields when another synchronized field changes.
Backbone checks periodically if fields have changed which are not recorded in the issue history.