Hihi.. nice to meet u..

Ya, I think I got this experience also when I seen the Stereogram, but I dont know why, I'm curious to know too.
And I think is the 'problem' of our glass. Are u a myopia ? for myopia the glass used is concave lens and the lens is designed for correcting distal object/focus, but not for near object/focus. When we seen at Stereogram, our eyes was diverging, I mean we relax all the intra-occular muscles just like what it did when looking at distal object, thus looking at distal object also help to relaxing the eyes. Then return to the concave lens, as we know the lens was for correcting distal focusing, when we seen a near object, our eyes must do some accommodation, that is narrower of the pupil and the eyes' len become more convect, more globular. Yet we are using the 'condition' for far object to seen at near object, so it will cause blurry.
I dont know whether what I 'guess' is correct or not, but, just for my very own openion.
My english language was bad and I hope you will understand what I mean, maybe I should draw a diagram in my explanation next time. 
All the best and good luck, to all of you.
Edgard