To be somewhat serious about the land of the dead - another plane is how it's best described. It's like the land of fey, or being in a fairytale. It's not consistent. I'm surprised it took you this long to figure out, actually. I wonder about the specifics- like does the forest just appear out of nowhere? Or does a forest already have to be there? I always the forest looked small from above, but the ground was completely hidden by treetops. Inside, it's much larger.
Also, I firmly believe Mictlan itself is constantly rearranging as well. The closer you get to the center, the more consistent things are. The outskirts are a mess. Roads are often longer or shorter than they should be, and only those who call it home can navigate it. "Navigate" meaning "control".