Think of the difference between photos and files -- liek the difference between a file and a letter - the date the letter was written may or may not be the same as the date it was saved
On your memory card of course the date the file was created and the date the photo was taken are the same - they both happened on the same date
But when you move the file to yhour computer a new files is created to hold the photo - that is created on the import date
Files contain data - it may be a photo, a letter, an excel spread sheet or any thing else - the finder works with files and show the file metadata - creation date of the file (nothing at all to do with the contents of the file), last modified date of the file, size of the file, etc
Photos on the other hand have different data about them - date taken, lens used, exposure setting, camera type, etc, etc -- this is stored as part of the photo in the EXIF fields or the IPTC fields - this is the photo metadata
Photo managers like iPhoto display the photo metadata -- file managers like the finder display the file metadata -- they often are totally different and there certainly is no reason to expect them to be the same
So to view photos and photo metadata use a photo program not a file manager
Another go at explaining it:
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.
When you export from iPhoto to the Finder new file is created containing your Photo (and its Exif). The File date is - quite accurately - reported as the date of Export. However, the Photo Date doesn't change.
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.