Where in the interface does it show which sites were tested? I couldn't find an option to display that.
The Box is there to test things for Ofcom to get a comparative view over a representative user base - the fact we get to see the data is in some ways an added bonus to thank us for our involvement. Ofcom is who define what should be tested working with Samknows. I'm guessing Twitch hasn't registered enough on their radar to test, they test Netflix and Youtube but don't test Amazon Prime Video which is a pretty big supplier as well.