Roku_Analytics LIB questions

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[/url:3ps9uaoc]
Users Overview
  Operating System
1.  iOS   
2.  Android  
3.  (not set)  
4.  Windows  

Thanks,
Steve