'Stereo' scenery

I hope some of the scenery gurus here can explain why this happens.After the recent threads about enabling 3D stereo in FS with red/blue glasses I decided to try it for myself.The effect is pretty good except for add-on scenery.If I fly over default scenery I get the 3D effect and can see the two red/blue images if I don't use the glasses. However if I go to any of my add-ons (mainly airprots) there is only a single 2D image but outside the add-on boundry the red/blue images are there.Just curious why this happens.TIA