Sign-Ups on the About pages is a User count. That used to be called, correctly, “Users” but was renamed recently to “Sign-Ups” in
https://github.com/discourse/discourse/pull/19020
As a simple User count, it doesn’t really track Sign-Ups, but counts the number of Users in a site as a proxy to the Sign-Ups, meaning that users who are deleted, never activated via the email link, etc are removed from the count.