Normally a device is placed in a speciffic class at creation time. Whenever you add a new device you will have to tell Zenoss in which device class you want to create it. Based on this device class, Zenoss will use speciffic modeler plugins and templates. Configuration properties including plugins and templates are being inherited by child classes from their parents and they can also be overridden at any level, including at device level.
Short answers for your questions :
- Most propbably one device has been manually created as a router and the other one as a switch
- Interfaces are created automatically during the discovery process. You should check if SNMP configuration on the device and in Zenoss match (version, community etc.), make sure no firewall is blocking your server from accessing the router and see if the correct modeling plugins are being used (most speciffically, zenoss.snmp.InterfaceMap)
Hope that helps