It looks like you have a double transform going on. This happens when you have two things influencing the translate of an object, and so both influences compound each other. In this case, you probably have a group of geometry, so the motion of cart (or cart group) is driving the arm forward, but the cart is also driving the joints' positioning, which in turn is driving the position of the arm, so you have two things moving the arm. A fix might be to ungroup or unparent the arm from the rest of the stuff, and so you only have the binding of the joint to the arm. Hope that makes sense. It's kind of hard to get your head around, but it makes sense when you think about what is determining the position of what.
It worked