The Origin Summary covers your whole website, meaning PageSpeed collects data for the entire user browsing session.
NitroPack only serves cached and optimized pages for guest visitors. When your visitors are logged in, NitroPack is automatically disabled for them. Naturally, this means that there are browsing sessions where your visitors are logged in, and which are not optimized by NitroPack.
For additional improvements, please look for a database caching tool which would improve the speed of your website without NitroPack. For custom advice, feel free to contact the NitroPack support team.