On the 650, the throttle is like a rheostat rather than a traditional throttle. The transmission gears down to keep it in the proper gear for acceleration. It took me a while to adjust to it but I now like as it keeps me from using the brakes as much as I did on the 400. The gearing down is very similar to how I ride my V-Star. Maybe that is why I never have to replace the brake pads on my bikes.
My 2011 Kia Optima does it to a lesser extent and people complained about it when the car first came out. It is rarely mentioned now.