Netflix uses an adaptive speed technology. Its bitrate depends on the quality of your network connection. I don't know for sure about Roku, but I suspect it does, too. They'll both vary depending on the time of day and how busy your local network is.
In other words, if you want the best reliable video and audio quality, you need to use "shiny discs."
In other words, if you want the best reliable video and audio quality, you need to use "shiny discs."