Flying Blue now has a variable pricing model. The mileage calculator returns the lowest, "starting from" price.
The most recent changes to Flying Blue [already more than 2 years ago: see this thread -
http://www.flyertalk.com/forum/air-f...ul-awards.html] means that there is "extended availability" for all members, meaning that they make extra seats available for redemption after these cheapest award seats have all sold out - but they charge extra mileage for them.
Are you sure you are choosing the cheapest option in both directions?