Because there is no transmission as such - Instead a sun planet gear system, the two motor generators and ICE are physically directly connected to the wheels. In an attempt to emulate a torque converter, the rolling forward or backward is really software. I too have noticed this in both my 400H and Prius and several hills around here I use my left foot on the brake pedal. Now that you have mentioned it, what I'm planning to do is find a hill and see how far back the 400H and Prius will roll backwards - I don't know if they will respond like a Torque Converter and only roll back so far before "biting".