Steps to reproduce:

1. Create a host, with an unavailable SNMP interface (for example, specify an unreachable address)
2. Create a SNMP item
3. Interface availability icon will show "Not available", which is correct.
4. Create the following internal item:   zabbix[host, snmp, available]
5. Create a second internal item:       zabbix[host, agent, available]
6. Test the two items

Result:

• zabbix[host, snmp, available]    returns  "0 - unknown"
• zabbix[host, agent, available]    returns  "2 - not available"

Expected:

According to documentation

• zabbix[host, snmp, available]    should return  "2 - not available"
• zabbix[host, agent, available]    should return  "0 - unknown"

The same applies to other interfaces (see screenshots)