Are you shooting "Raw"? Then what you are seeing is not adjustment of the photos, but the replacement of the camera generated previews by the new previews generated by Aperture from the raw images.
Apertures own previews will show exactly the naked raw format and none of the in-camera processing, that is applied because of settings like "Preserve Highlights". These are only shown in the jpegs that the camera creates based on its settings and automatic programs.
If you want to keep the enhancements that you see in the original preview when the images is imported, you may want to shoot jpegs and not raw. only the jpegs will profit from your in-camera image processing.