The number of unsupported items in database, as counted by internal item zabbix[items_unsupported] is not the same (is about 3 times) the number of items listed in unsupported state in Configuration->Hosts, selecting the "items" link of an host and then clearing all the fields and selecting "unsupported" in "Status" dropdown.
The resulting URL is something like http://ZABBIXURL/items.php?filter_hostid=0&filter_group=&filter_type=-1&filter_value_type=-1&filter_status=3&filter_hostname=&filter_delay=&filter_with_triggers=-1&filter_application=&filter_history=&filter_templated_items=-1&filter_name=&filter_trends=&filter_key=&filter_set=1

For example, now I have 395 items in "Status of Zabbix" pane (which corresponds from what zabbix[items_unsupported] reports), while I see a list of 122 items in the above link.