Matching images produce diff

[limscoder]

I have 2 images that are identical when compared manually with graphicksmagic, but webdrivercss outputs this diff:

The smeared diff on the right is white pixels in both images. I've seen this pattern a couple of times. Any idea what's happening?

These are the images being compared:

Here's the output when I compare them manually:

[BaseInfinity]

I've also run into this, I've had to disable a test completely because of it