Most of autostereograms are meant to be seen wall-eyed, instead of cross-eyed [0]. With wall-eye, you shift the convergent point being what you're seeing, while with cross-eye you shift it before.

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.

[0]: https://en.wikipedia.org/wiki/Autostereogram