WP-Cron is slightly different to a system cron in that:

• WP-Cron uses intervals – whereas a system cron is triggered at specific times
• WP-Cron only works when a page on the site is loaded – if there is no activity on the site then WP-Cron tasks will not run until there next is a vistor. Not useful for sites that are not really busy. However it can be worked around see here.

Resources

https://developer.wordpress.org/plugins/cron/scheduling-wp-cron-events/

https://tommcfarlin.com/wordpress-cron-jobs/