Does iPad rotate right sooner than it rotates left (or up or down)?
I've just tried with an iPad (3rd gen) running 5.1, there doesn't seem to be a difference when I tilted left or right. At least, not a 20° difference.

But there could be. Since starting from iPad 2 there's the Smart Cover, people are more likely to tilt the iPad to the right, to have it in "typing mode" so Apple could have lowered the angle necessary to rotate the screen.