And I find that, hilariously, my visual system actively fights the merging of the boxes. When I successfully look at a real stereogram, there is a point at which my eyes snap into alignment and the stereogram pops out. In this case, it's like the opposite: I try to steer my eyes to the point where the blue and yellow boxes overlap, but if I get close they skitter apart. It's like trying to bring two identical magnetic poles together.

I'm sure this stuff is highly idiosyncratic. Everyone's visual system is different. It develops via visual feedback at early ages and so is different for everyone, sometimes to one's detriment:

http://www.lazyeye.org/