The physics has two modes "fixed" (default) and "Framerate Independent".
With "Fixed" 1/60 is used instead of dt. This is the default and useful to have similar results from run to run. The side effect is if your fps dips below 60 then the physics objects will move slower.
For example normally if the velocity is set to 100 and the fps is 60 then 1/60 and dt will be about the same and the speed will be 100pixels/second.
On the other hand say the fps is 30 then dt will be about 1/30 or twice as much as 1/60 so the speed will be 50pixels/second.
With your result of 0.833x I'd guess your fps is about 50.
You can set the stepping mode to "Framerate Independent" with an action to make the physics use dt instead of 1/60. With a stable fps of 60 you'll not notice much of a different between the modes. The simulation can become unstable if the fps varies too much but the speeds will remain the same at 100pixels/second. At least until the fps drops below 30fps. After that 1/30 is used instead of dt. It would seem that is the minimum timestep that will be used. The result is slowdowns again much like "fixed".
Anyway that's my breakdown of what's happening and here's my test capx:
On another note, AnD4D Colludium I think this info is relevant to the topic you made here:http://www.scirra.com/forum/physics-forces_topic85125.html
In summery setting the stepping mode to "Framerate Independent" will help with your situation and keep the speeds matching since your fps is in the range of 30-60. If your fps drops below 30fps then all the physics objects will slow down.