I'm guessing that happens because Windows computers are not a device, but a Windows machine.
So a Windows computer is a Windows Computer, everything else is a device. A monitor is a device, a VMWare server is a device, a switch is a device, a printer is a device, a firewall is a device... you get the ideia...
If you check the reports, you have something like:
Computer: New computers found in the last 24 hours
Device: New devices found in the last 24 hours
See the difference ?