I've had no balance issues on bikes, I've been riding bicycles since I was 6, and motorcycles from the age of 12. When riding on the road, I balance and change direction by shifting my weight, and this shift is done by my mid to upper body. On rollers, subtle shifts in weight result in large lateral movements, so I tend to shift weight lower down, in my hips. Roller workouts seem to work this into muscle memory, and when I get back on the road, I find that my hips and lower body are unconsciously working to maintain balance, making me steadier without any other effort. It's hard to explain, but I've always been able to ride a bike steadily, but rollers make it rock steady.