Based on some JPL documentary videos, I recall the engineers involved intentionally over-specced the components on the hopes that the mission would be later extended to go further out into the solar system. Check out the JPL channel on YouTube. There is a video series about the different missions throughout NASA history. Its really worthwhile watching.

https://www.youtube.com/playlist?list=PLTiv_XWHnOZqFnWQs393R...