if I select multiple images to apply a change to all Aperture it does so only with the first selected. Why?

You probably have Primary Selection only on.

Aperture identifies the images you’ve selected by displaying them with a white border. When you select a group of images, the actively selected image, called the primary selection, appears with a thick white border and the rest of the selected images appear with thin white borders.

Primary Only button: Click this button to make changes to the primary image selection only.

Glad it helped.

For future reference you should know that there is a community here dedicated to Aperture see Aperture

regards