Jeff, AP gave you a great scenerio with the site pins in the same plane ie 20, 40, etc..

Think of it this way as well, your bow is set up with the site pins to launch your arrows up, actually your holding your bow so that the arrow leaves in an upward trajectory to counter act gravity so your arrow arcs to the target. Do you agree with this?

Now, if your canting your bow off level, your actually shooting the arrow up and left/right depending on the direction of your cant.....do you agree that your arrow would hit left or right depending?

Without a level, if you cant your top limb towards the upward direction of the hill, you'll be leveling your bow more so because instinct tends to be off level on side hills. Practice this at home, get on one of your side hills there and draw your bow ignoring your level, anchor in and check your level...I'd bet your off. Now level your bow and watch how hard you think your canting into the hill to level your bow.

Now, levels can be misaligned when shooting extreme uphill or down hill shots because the level actually changes planes. If you hold your bow directly over your head, your actually leveling left to right vs, ATA. Comprende?