OS Fingerprinting on Ruckus is based on DHCP requests hence i think couple of reasons why it would not work:
1. Use of Static IP.
2. If OS info in DHCP requests from Chrome device are blank hence you see this.
3. If Junk OS info in DHCP requests from Chrome device which not known to Ruckus system may also cause this.
If your issue falls into last two then you may have to check further like trace the DHCP traffic to see whats chrome is sending to be sure.