As far as why one house is perceived as secret, I think it is because the space in Shrek was reserved, but they were deciding between the 10th house or second show. They ended up passing on the second show, but after the initial 9 houses were planned. The late decision and the other changes/misinformation made it seem more secret than it was planned to be. The initial press release touting 9 houses shows how late the decision was made. Was the content of the 10th house pre-planned? Was it decided later? I don't know, but some scenario like this seems likely. I based this on posts made here and HNN.