The documentation on SNMP trapping states: "For each trap Zabbix finds all corresponding SNMP interfaces on hosts for the received IP or DNS address".

The "or" in this wording is ambiguous. It lead me to believe that either match is fine. It turns out that if you specify a name after ZBXTRAP, it only matches if the interface is configured to use the name.

https://www.zabbix.com/documentation/2.0/manual/config/items/itemtypes/snmptrap