I've noticed that if you bridge over the top of an ocean and go to the edge of the world, you can see things spawning on-screen due to how the camera stops panning near the border; however, if you were to stand in the middle of a standard volcano farm and use the binoculars to see past the edge of your screen into the spawning area, you cannot see enemies spawning, even though you yourself do not move. I have to assume that when using the binos, the "You" the game is calculating the spawn distances from is not where your character is, but where your cursor is.

Does that make sense?