All of these are actually optical illusions and psychology. The article mentions the "larger, steeper" hill that dwarfs Spook Hill but it is that larger and steeper hill just beyond that obscures the fact that Spook Hill is actually still climbing although it looks like it is going downhill. Judicious (or sometimes, accidental) placement of false plumb lines such as telephone poles or fence posts at consistent angles that are mistaken for straight up-and-down, add to the illusion.