Extention enable PHP
Extensions can be loaded into PHP two ways. Either statically at compile time, or dynamically at run time.

Statically loaded extensions are included in the PHP binary at compile time, and does not require an extension= line in php.ini to load it.

Dynamically loaded extensions are not included in the PHP binary, but are saved as dynamic link libraries (.so files on OS X). They require a extension= line in php.ini to load it at run time.

The short answer to your question: Your cURL extension is statically compiled with the PHP executable, therefore it works even though you haven't got a extension=php_curl.so line in php.ini.