The metadata inside of some image formats contains a thumbnail (or stamp). This information is entirely separate and distinct from the actual image itself, but is formed when the photograph is taken.
Not all editors deal with stamps (thumbnails) or not all editors deal with them properly.
When I wrote the Super Simple Photo Resizer, I came across this issue, and instead of simply not dealing with it, in at least some case (IIRC) I delete the stamp/thumbnail from the image file entirely. There are very good reasons to do this...
Imagine you're at home, getting out of the shower, and your spouse snaps a photo of you. While you don't want to post full frontal nudity of yourself, perhaps the facial expression is really funny, and you want to share that. So, you crop the photo and email or upload it...
Now, if the software didn't erase the stamp, you've just sent that original stamp of your naked body around. Not good. And probably not what you'd intended to do.
Anyways, that's a bit long for the simple explanation. -- Stamps and image data are not the same.