The distinction is not really based on OS, and instead on the monitoring protocol. Interface monitoring for WMI nodes is rolled into the node element license when you have SAM.
Reason: Under NPM will list: The interfaces monitored with SNMP, and under Manage Nodes it list all the polling options.
You can validate by:
Log on to the SolarWinds Web Console with an administrator account.
Click Settings > All Settings.
Click License Details in the Details section.
Under NPM will list: The interfaces monitored with SNMP
(And does not include the Interfaces that are under Monitor by other method)
You can confirm the number on Orion Web Console > Settings> Manage Nodes and filter by Polling method and under search select Interfaces instead nodes.
You can setup a report to display all interfaces, and add filter by SNMP monitor
