What’s the difference between "zend_extension" and "Extension" in php.ini?

Question

I noticed that some extensions like Opcache can only be activated (added) via zend_extension, in accordance with http://php.net/manual/en/opcache.installation.php:

Platforms "not Windows":

zend_extension=/full/path/to/opcache.so

Windows:

zend_extension=C:\path\to\php_opcache.dll

I wonder if there’s any special reason for this?