When changing the default email digest frequency and confirming to make the changes to accounts retroactively, the changes do not take effect for all accounts. Clicking Yes to the following question does not have the expected behavior: "Would you like to apply this change historically? This will change preferences for n existing users.
While some accounts may update to the desired frequency, in my experience many do not. Possibly this has something to do with having selected “Disable summary emails for all users.” at one point. Users who’s frequency is not updated are users with Summary emails not enabled within their user preferences. They did not make these selections themselves as these users were created as part of SSO with Wordpress and these accounts have never been logged into.