A daily summary is unusual. When a Discourse launches, it is in bootstrap mode until there are 50 users. Only in bootstrap mode are summaries sent daily.
Bootstrap mode is turned off after the site reaches 51 users; @techapj can confirm but as I recall, the summary frequency is set back to weekly at that point for all users.
Anyway, thanks to @sam’s excellent work unsubscribing from the digest is a one-click affair if people are uninterested.