Are you assuming a constant force input? Think of F=kX over a broad range of frequencies, with k being your dynamic stiffness. if X increases with a constant F, the k has to decrease. Or maybe you are thinking of something different?
i think we should know which is the reason and which is the result. K is the property of materials or components, we can get it by testing force and displacement. of course, the force can't be constant in the experiment. that's to say, the force will increase with the increase of displacement, but their increasing rate are different. so K is different.but i can't explain why K get smaller at higher amplitude in dynamic test.