ok, I figured it out....

the "one way fee" is a fix amount and it will be divided by the
number of rental days. That's why when I bring the number of day down the "one way fee" goes up and if I bring the number of days up, the price comes down. Wow, why not just put the total instead of per day.

Another thing I found, if I return the car in the same country, let's say France, the price for "one way fee" comes WAY down.