Instead of the bubble "rolling over" on a level plane, it's twisted 45 degrees, meaning one end of the level is higher than the other, making it appear the bubble isn't level.

Imagine a 3 deminsional rectangle. Perfectly level to the ground. Let's call the end on the ground side 1, then the next side 2, and so on.....So you're on side 1, and you roll it halfway between side 1 and 2. If that rectangle doesn't remain perfectly level, the bubble will lift to the high side. Thus creating the illusion that the bow isn't square. So you torque the bow to move the bubble back to level, and what you've actually done is moved it OUT of level, thus shooting right or left.

Man, I know exactly what I'm trying to say, but I'm not saying it very well........Wish I could draw a picture...