The timing for a number of Magento's core cron tasks such as sitemap, stock alert notifications and log cleaning can be adjusted via the admin's System Config. This means they are not defined inside the
cronjob node of a module's config.xml however they do get written into the
cron_schedule table like other jobs.
How does the cron scheduler find those jobs? My guess is that there is an event thrown during the schedule generation, but I can't see any instances of
dispatchEvent within Mage_Cron.