Found this in my own lab. Had a bad ethernet cable but system-plugin was reporting interface as 100% Available.
Steps to reproduce:
1.) Make sure agent is monitoring a network interface
2.) Disconnect network cable (in VCenter, this can be done by unchecking "Connected" for interface)
3.) Watch what system-plugin reports for Network Interface. It will show as 100% Available
50% Availability reported since interface is still technically up, just not fully Available