What's different about the pasted part of an image?

[Corelius]

Take a 500-pixel-wide image (for example). Crop it down to the left 200 pixels and copy that image to the clipboard.

Now undo the first crop, and crop the image down to the right 300 pixels.

Now enlarge the canvas to 500 pixels wide, but leave the (original) right 300 pixels as the left 300 pixels.

Now paste the (original) left 200 pixels into the empty right 200 pixels of the image.

If you tile that as Windows wallpaper, you'll see the original image in full several times (although not tiling neatly from the left side of the screen).

So far so expected. But here comes the weird issue.

Now apply local tone mapping to the image to a degree that makes a clearly visible difference. And then tile that as wallpaper.

If your PSPro works the same as mine (I'm using PSP X9), you'll see a visible line in the middle of the image where the (original) left 200 pixels meets up with the (original) right 300 pixels, because, for some reason, the local tone mapping has a somewhat different effect on the part of the image that went through a copy-and-paste (the 200 pixels) than it does on the rest of the image.

Anybody know why this is, and if there's any way to avoid it?

I get the same glitchy result whether my paste is a Paste As New Selection or a Paste as Transparent Selection or a Paste As New Layer.

[Corelius]

I think I've figured out the answer to my own question.

It makes sense that, unlike many image-processing effects, the effect that Local Tone Mapping has on any given pixel will be dependent on the pixels in its vicinity. (Kind of a duh if you think about it.) So when a pixel that used to be in the middle of an image ends up at the edge of a cropped version of the image, it won't get modified in quite the same way as the originally-positioned version of that pixel.