I am not really sure. This is what I suspect but have nothing to offer to qualify these suspicions. Speedos on bikes are not accurate by design because they cannot be consistent. I would think that for liability reasons, the manufacturer calibrates them to "read" faster than what they are actually going, erring on the side of caution, With a car, the speedo is calibrated based on the distance the vehicle will travel with each rotation of the wheel. That remains consistent because the wheel rotates on the same surface, the outside circumference of the tire. That always remains the same except for wear. Your speed is based strictly on distance and time. But with a bike, we all know that when cornering, we are not riding on the outside circumference of the tire. We lean into corners. Well most of us do. So to some degree, that wheel has to rotate more often to realize the same ground speed as it would be riding vertical. So even though your speedo reads one thing, your actual ground speed is something else. Again, the speed is based on distance and time. But because your bike is riding on part of the sidewall when cornering, the circumference measurement changes. It reduces which either reduces your actual ground speed, or increases your speedo speed if your ground speed remains the same. If you really want to see the difference, sit down with a tape measure and measure the two circumferences and do the math. I think you will find the difference quite surprising. BTW, that is what happens when you change tire sizes on your vehicle as well. For speedo accuracy with your car, stick with stock tire sizes.