This was a huge trend in the 90s that I missed out on, because for some reason I can only see the cross-eye versions, but all the books and posters you could buy were the straight-eye versions.
Either can look in front or behind.
So it creates the exact opposite 3d. Watching a stereogram meant to be seeing wall-eyed, cross-eyeing you'll see an upside down 3d. Some stereogram are mean to be seen cross-eyed though, which is considered easier to see.
Here "being" probably should have said "behind".