Core Web Vitals are not always available for all the URLs.
Ref: https://support.google.com/webmasters/answer/9205520?hl=en
The report is based on three metrics: LCP, FID, and CLS. If a URL doesnot have a minimum amount of reporting data for any of these metrics,it is omitted from the report.
"Page experience on mobile" report is based on the URLs for which the core web vital metrics have been measured (field data) by Google. Also, it seems only those URLs with field data show up on the "Good Page Experience" drill down in the "Performance" report.
Note: In my case, I compared the total impressions of Good URLs on the Page Experience report and the total impressions from Good Page Experience drill down. The numbers are very close.
