It wasn't mentioned in the first post how long the rental was for but you could be tinkering in and out of a "minimum keep" issue, a weekend special, a week special, a three day special, or something similar.

The computer calculates the "Best Rate" for the dates and times you imput. Once you stray out of those perameters then the rates change, sometimes dramatically. AND...if your actual Rental strays out of those "times" your price can change too.

A Hint...
When putting in your times for rental...always use your drop off time...NOT your airline departure time. Many times I will see someone's rate change on them, kick them out of a Three Day or Eight Day minimum keep requirement on an internet special because they only had a three day or eight day rental because of an hour or two as they entered their departure time of their flight. When we process the rental, and they are returning at the same time as they picked up, their special booking rate is gone and the customer is mad as a hornet. Not our fault...they aren't keeping the car the same amount of time as they entered in the computer!
Save yourself some grief...be specific as you can so there are no surprises!