I'm so glad you noticed that (& so sorry it took me this long to notice this thread, oops!). I've seen various kinds of snakes do this too, not only rat snakes, and
it makes sense, because their eyes are mostly off to the side of their heads, not facing forward like our own.
You know what else is interesting? I forget where I read about it, but apparently snakes can tell direction (like, of prey) by the 2 separate forks of their tongue-
one fork will get more scent than the other, thus the prey is "that way!"Snakes are cool... & most people don't know the half of it. They focus on what snakes don't have, instead of what they do.