As I understand it, you select the beta program that you wish to enroll in and then that will auto update.
For example, when I right click on WMR for SteamVR and select properties, then Betas, it gives me an option if I want to opt into the "beta - public beta" or "lkg_release" or "none".
My point about the resolution numbers vs the percentage bar is that, I believe, depending on which beta you select, the bar will show a different scale. ie : 100% in one beta will give be a different resolution than 100% on another beta.
....just more variables to confuse everyone and invalidate comparisons!