It's very common for airlines to do this. They aren't transparent about the taxes at all. I've tried booking using FF miles with KLM several times, and have often been quoted an extra GBP100 or so in combined taxes to book a return flight compared to the price of the two legs separately.
When I queried this with KLM I was told that was the way taxes work, and couldn't get any further answer out of them. I would love to have a detailed explanation of exactly how tax charges are calculated.