Roku_Analytics Google options (Operating System, etc.)

I am successfully posting to Google Analytics, but the Operating System is not set...?
How does Google determine this?
I would LIKE the (not set) to be ROKU!!!
Example from https://analytics.google.com/analytics
Users Overview
 Operating System
1.  iOS   
2.  Android  
3.  (not set)  
4.  Windows  
Thanks,
Steve