That's how I experience mental visualization as well. It's as if there is one framebuffer in front of my head (what I see in the real world, or inside my eyelids when my eyes are closed), and a separate imaginary framebuffer that feels like it's located above and behind my head. Is yours also located behind your head?

An interesting effect of some psychedelics is that they can cause my front framebuffer and back framebuffer to merge into one framebuffer. It's quite a strange experience. I wonder if anyone else has experienced this as well!