Declined events
Google Calendar keeps declined events on your calendar. CalendarBridge syncs declined Google events with a “Declined:” prefix prepended to the subject.
Google calendar keeps declined events on your calendar, but has a setting to hide them:

CalendarBridge cannot see whether you have chosen to hide declined events. Thus, calendarbridge will sync declined Google events with a “Declined:” prefix prepended to the subject of the event*. If you subsequently delete the declined event from the source calendar, the “Declined:” event copy will be deleted.
*Except for iCloud destination calendars; copies of declined Google events on an iCloud will not have “Declined:” prepended to the subject.