WP-Cron is not a real clock.
WordPress normally checks scheduled tasks when a request reaches WordPress. On a low-traffic website there may be no request at the time a check is due. On heavily cached or CDN-served sites, many visitor requests may never load WordPress at all.
That means a task scheduled for 10:00 can start later than 10:00. Site Watch cannot make WordPress execute a local check until WordPress itself gets a chance to run its scheduler.