That's called adverse yaw. The down aileron has more drag than the up, especially in high angle of pitch, which causes the rolling harriers to turn away from the direction of roll. But wouldn't any differential be canceled out in a rolling harrier since it would be positive upright and negative inverted? If you're rolling right and have less down on the left aileron and more up on the right, then when it goes inverted, it will have more down on the inside aileron and less on the outside, thus canceling out the gain from the previous half cycle.