So to create a stereoscopic image, the two combining pictures have to have minor differences for the illusion to work right? This can be a simple perception change or moving a couple of objects around (I don't know the science behind it, this is just what I've picked up from reading here). For instance the raven, crow, whatever in Justin's pic is hidden behind the curtain less so in one picture and more so in the other.
Where am I going with this? Well when you look at the bird through the glasses you always see the image that shows more of the raven. The glasses show more detail. This holds true for every object that I've found that is in a different position between the two images; the view that reveals more is always shown when looking through the glasses.

I know this is almost definitely looking into things too much so I was just wondering if anyone could explain the science behind this. Cheers.

The pictures are taken from two different angles, just like your eyes would see the scene naturally. Look at something that's in front of you right now, cover one eye while looking at it, now uncover that eye and cover the other. That's the same slight difference you see in the pictures.

Ahh silly me. Thanks.

Not at all, you're welcome.

yeah he's right. i think it is called parallax effect. It is the same way astronomers measure the distance of some stars in the universe.