What happened before this problem occurred? Have you moved your iPhoto library? Upgraded software or restored your system? Or have you accessed your iPhoto Library from other software? Where is your iPhoto Library located? In the default location in your "Pictures" folder or on an external drive?
Without knowing exactly what you have done and exactly when this happened it is impossible to know what happened
The most likely is an error in a past transfer (like importing one iPhoto library into another one - did you ever have lots of duplicates to deal with after an move to a new system? If so how did you handle that? And if so do you still have access to the iPhoto library form before the transfer?)
I've done a million things, new laptops, new operating systems, new versions fo iphoto since then.
If you look at the folders inside your iPhoto library (ctrl-click the library and select "Show Package Contents") youls se, that iPhoto keeps several image files for one single photo (for example in iPhoto 9.4.2: the original image file, lower resolution previews of the edited version, thumbnails). It looks like in your case iPhoto were using the previews or thumbnails instead of the original mater image files.
As Larry said, that can happen, if you import an iPhoto Library into another library. How exacly did you restore you iPhoto libraries, when you setup your new Laptop or upgraded the OS?