Thanks for posting this question.
Based on my understanding of your issue description -- the Memory percentage seen on the App Service Plan (ASP) vs the memory utilized on the Web App level would be different due to system processes.
Just to highlight, it’s the whole memory which you see at ASP level. This includes all the memory used by system processes and application processes.
Furthermore, you may leverage Memory analysis tool from diagnostic blade to fetch drill-down view of
--You may leverage App Service diagnostics from Azure Portal> Navigate to your App Service app in the Azure Portal. (screenshot below)
-In the left navigation, click on Diagnose and solve problems
- Review - “Availability and Performance” >> “Memory Analysis”
--Memory analysis tool to fetch more insights on: App Service Plan Density, Private Bytes, Physical memory, Memory drilldown/Per instance/app.
--Average memory working set: The average amount of memory used by the app [in megabytes (MiB)].
--Memory working set: The current amount of memory used by the app (in MiB).
Reference: Monitor apps in Azure App Service
Kindly let us know, I’ll follow up with you further.