I have found that pics taken with my Nikon D80, when looking at the original file in the Finder (in iPhoto, File/Reveal in Finder) display the time and date the photo was taken. This time is also reflected in the Info pane of iPhoto just below the camera info.
If I have modified the photo in iPhoto, and Reveal in Finder-Modified File, the Finder info reflects the date and time modified.
There are two kinds of metadata involved when you consider jpeg or other image file.
One is the file data. This is what the Finder shows. This tells you nothing about the contents of the file, just the File itself.
The problem with File metadata is that it can easily change as the file is moved from place to place or exported, e-mailed, uploaded etc.
Photographs have also got both Exif and IPTC metadata. The date and time that your camera snapped the Photograph is recorded in the Exif metadata. Regardless if what the file date says, this is the actual time recorded by the camera.
Photo applications like iPhoto, Aperture, Lightroom, Picasa, Photoshop etc get their date and time from the Exif metadata.
The problem is that the Finder doesn't work with Exif.
So, your photo has the correct date, and so does the file, but they are different things. To sort on the Photo date you'll need to use a photo app.