Office365 Active User Report

LastSignInDate only reflects Interactive logins, as in the user performed a full sign-in (entered credentials or used whatever primary auth method they have configured). With the OAuth model most applications now use, the majority of sign-in events are non-interactive ones, where the client presents a valid refresh token to continue accessing a given app/service. In some cases, even months after the last "interactive" login, as long as the refresh token is valid.