Steps to reproduce:

1. Add host(s) with Active Agent checks only (agent connects to Zabbix server, server can not connect back to the agent
2. Navigate to dashboard and view Host Availability widget
3. Number of hosts shown in Host Availability widget will not reflect hosts added in step 1

Result:
See screenshots: in this example, there are three hosts available, and you can see them in the hosts list. However the Host Availability widget only shows one host
**

Expected:
Host Availability widget should reflect all hosts, regardless of whether they are active or passive agent checks

Discussion here: Active Agent hosts not showing in Host Availability widget - ZABBIX Forums