The general answer is that it's the difference between negative feedback and positive feedback. The specific answer is that it's because of weight distribution.
Unless the yardstick (or top marker, in my case) is perfectly symmetrically balanced on the two rolls, one side will be hanging slightly further out than the other. This side will be pressing down with more force on the roller. Therefore, that roller will have a greater effect when it rolls.
If you roll inward, whichever roller is dominant will push the stick slightly towards the other roller. This will have the effect of increasing the weight of the stick on the other roller. This will cause that roller to become dominant. This will cause the stick to roll back slightly the other way. This is negative feedback, and will keep the system in balance.
In contrast, if you roll outward, which ever roller is dominant will pull the stick towards it, which will increase it's dominance, and the pulling effect will strengthen. This is positive feedback.