I think you can think of it as "assuming that the user does not start interacting with the app, when should we measure the app memory usage". We want to measure it when "everything" is loaded, and by everything I mean everything that is part of the startup path (including background data like awsomebar data for browser and metadata for contacts).

Of course if the user will start interacting with the app before that, we'll never reach that stable point and I think it's ok.