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 that aren't optimized by NitroPack because your visitors are logged in.
For additional improvements, please look for a database caching tool that would improve your website's speed in these instances. For custom advice, feel free to contact the NitroPack support team.