Eternal power denotes the divine attribute of power, which is not originated; originated power denotes a creature's limited ability. The shared word “power” does not make their realities equal.
A human may voluntarily act and acquire an act, yet the human and the capacity to act remain dependent on God. Human ability is therefore not a negation of divine power.
A person chooses to write and moves the hand. The existence of the act is considered under creation, while responsibility is attributed to the human voluntary agent.