From the source code it's calculated in the following way: ref: http://docs.ckeditor.com/source/event.html
/**
* Gets a number represeting the combination of the keys pressed during the
* event. It is the sum with the current key code and the {@link CKEDITOR#CTRL},
* {@link CKEDITOR#SHIFT} and {@link CKEDITOR#ALT} constants.
*
* alert( event.getKeystroke() == 65 ); // 'a' key
* alert( event.getKeystroke() == CKEDITOR.CTRL + 65 ); // CTRL + 'a' key
* alert( event.getKeystroke() == CKEDITOR.CTRL + CKEDITOR.SHIFT + 65 ); // CTRL + SHIFT + 'a' key
*
* @returns {Number} The number representing the keys combination.
*/