The problem lay in a crucial difference between a missile and an economy--which is that, unlike the people who make up an economy, the components of a missile do not try to understand and anticipate the forces being applied to them.
Hence, although a given propulsive force always has the same, predictable effect on a ballistic missile, a given policy action--say, a 25-basis-point cut in the federal funds rate--can have very different effects on the economy, depending (for example) on what the private sector infers from that action about likely future policy actions, about the information that may have induced the policymaker to act, about the policymaker's objectives in taking the action, and so on.