Treadmill with "this device" pace incorrect

When I use "this device" in the treadmill my pace is twice as low as it should be. For example, if my actual pace is 6/km, Rolla will show me a pace of 3/km. I noticed that the faster I run, the more accurate my pace is. Unfortunately, for casual running that takes place between 9 and 12 km/h, the pace is quite inaccurate.