From the documentation (https://www.zabbix.com/documentation/3.4/manual/web_interface/frontend_sections/monitoring/overview):
The Monitoring ? Overview section offers an overview of trigger states or a comparison of data for various hosts at once.
In "Overview", Zabbix displays "OK/Green" state for triggers with "Unknown" status which is misleading. Zabbix should probably use a different color to highlight them (e.g. white/grey). Also, it would be great to be able to hide them by using a filter.