Devices with unknown OS in Fortigate

Hi,

I have a Fortigate 90D in our network. The problem is when i check in Device Inventory about the devices in our network, i see many devices with UNKNOWN OS tag. The windows OS devices have windows icon with them, same is the case with android and Mac OS devices. So what are these devices with UNKNOWN OS and how can i get to know about these devices in my network?  I want to know why Fortigate is showing Devices with UNKNOWN OS?

Thank You