The key here is that you returned it at a different location. That's ALMOST ALWAYS going to cost you more. You can return the car at the same location earlier and it will not affect your rate (usually goes down). However, because you decided to return it to a different location they change the contract terms on you.
Example I had to deal with...
Extended my rental out of IAD to drive to NY because we got permission to leave a day early instead of flying the next day around some holiday. When I got to NY I had another rental lined up out of ISP but when I called Hertz to ask them what the deal was if I was to return my car at ISP they told me that my contract would be nullified and I would be charged 37 cents a mile in addition to my contract rate. Considering that I had put something like 700 miles on the rental it would have been something a little over 600$. I told them, forget this I'm going to drive it back to IAD and return it on monday and rent at IAD instead of ISP. In the end, it cost me less but I had to put up with more BS.
I think you can beat the system (especially if your CDP/contract allows for no fee one way rentals) by simply returning the car and making a reservation from Point A with a return at Point B because that will be your standard rate vs the randomly selected more expensive Hertz rate they come up with. I haven't tried this yet however so YMMV.