Photoshop has problem with .JPEG decoding ??

Question

There seems to be an issue with either JPEG decoding in Photoshop or the blend modes don't work as they should. Or maybe someone finds the true reason ... ?
Let's go step-by-step:
This is the original image file (JPEG):
This is the same image converted to .png:
I examined the differences. Open one image and add the second image as a new layer over it with blend mode = difference. Use magic wand tool, with tolerance=zero. Expected results: entirely black image, entire image selected. Actual esults (white filled selection):
[IMG]http://img372.imageshack.us/img372/694/differencepg1.png[/IMG]
Merge both layers, invert the colors and you see the differences between the images. I applied a curves adjustment to exaggerate the differences:
[IMG]http://img241.imageshack.us/img241/1995/difference2zw5.png[/IMG]
Again, this time you should see an entirely white image, if both images were identical.
The JPEG-to-PNG conversion was done with a third party image viewer that I trust. Obviously, the possibility exists, that this conversion is not exact/losless. But I can refute this, as I converted both the source JPEG image and the converted PNG image to BMP images, and the two files that were obtained this way were bit-for-bit identical:
So it's impossible that any loss occured, given that in the a losless reconstruction was possible.
Thus the error must be somewhere within Photoshop.
Any ideas?

Answer 1

There seems to be an issue with either JPEG decoding in Photoshop or the blend modes don't work as they should. Or maybe someone finds the true reason ... ?
Let's go step-by-step:
This is the original image file (JPEG):
This is the same image converted to .png:
I examined the differences. Open one image and add the second image as a new layer over it with blend mode = difference. Use magic wand tool, with tolerance=zero. Expected results: entirely black image, entire image selected. Actual esults (white filled selection):
Merge both layers, invert the colors and you see the differences between the images. I applied a curves adjustment to exaggerate the differences:
Again, this time you should see an entirely white image, if both images were identical.
The JPEG-to-PNG conversion was done with a third party image viewer that I trust. Obviously, the possibility exists, that this conversion is not exact/losless. But I can refute this, as I converted both the source JPEG image and the converted PNG image to BMP images, and the two files that were obtained this way were bit-for-bit identical:
So it's impossible that any loss occured, given that in the end a losless reconstruction was possible.
Thus the error must be somewhere within Photoshop.
Any ideas?