To understand automatic updates of the WordPress Core you need to understand the version system for WordPress itself.
Understanding WordPress Versions
There are 2 types of WordPress updates:
Whether one update is major or minor depends on the current version of your site, and the version of the update available.
For example, consider WordPress version 3.8.1
The minor version of this is "1"
The major version of this is "8"
So, when version 3.8.2 is released, the only number that changed is the minor version number. So this would be considered a minor upgrade (normally bug and security fixes).
But, if the next version you have available to install is "3.9.0", since the major version number increased (regardless of the minor number), this would be considered a major upgrade (feature releases and major changes).
Automatic Updates For Major and Minor Versions
When WordPress.org released the automatic updates system, they decided to only set minor versions to be automatically updated.
There is currently no way within WordPress to set the automatic updating of major releases without some programming - or with the WordPress Shield Security plugin, for example.
To learn how to set-up the WordPress Core Updates option within the Shield plugin, read the article here.