If the contract for the Falcon is anything like what I have been involved in, there would have been a flat yearly charge irrespective of the hours flown that covered up to a certain numbers of hours. After that number of hours were flown in that year, a rate/hr would then apply.
If it is a calendar basis, the Govt may be using up the hours already paid for under the flat yearly charge. Therefore it is not really costing us anything extra.
Just guessing.