You've given it an image with a white border, and expect it to know that the white pixels aren't desired. It can't really know that.
You could put the image on a layer, use the Magic Wand tool to select the white, expand the selection just a little, then delete it. Content Aware Fill knows that transparent pixels aren't candidates for being used to reconstruct parts of the image.
Or you could use layer masking. Similarly, Content Aware Fill won't use parts of the image that are masked off to fill other parts.
Hope this helps.
aaieee how could I have missed that? Please dont answer.
I'll answer anyway: We all have things like this happen. It's challenging to keep all the details straight all the time. If this stuff was easy anyone could do it.
Happy I could help.