The left and right images are interchanged to give a correct stereo view or a pseudo stereo view.

parallel eyes

In the above pair, the left image is on the left.
If you instinctively cross your eyes to view it, the result will be pseudo-stereoscopic.
Compare it with the image below.

cross eyes

Now the right image is on the left and may be comfortably viewed 'cross-eyed' by most people.

Sometimes the displayed stereo image will not look 'quite right', try pressing 'X' and the difference should then be obvious.