So the Activity Summary emails by admin settings default are weekly and each new account will have ‘weekly’ set as the frequency when created.
Adjusting the admin setting only affects accounts created after the change.
I think it would be better to have ‘default’ set as the frequency for each individual account. So unless they specifically change it to say, weekly, then the period used will always be the same as the current forum default.
Benefit would be allowing the admins to have more control over the Activity Summary emails frequency and allow adjusting it over time for the significant amount of users who use defaults. While not affecting users who have set it to something specific.