"The A/C is probably on a non operational lease until it goes into active service and being used as what we called a "hot spare" until they include it in their schedule. They would likely be paying a much lower lease rate than if they were using the A/C so in the grand scheme of things its possibly just loose change to them. The lease will change to an operational lease once they start using the A/C and then they will be billed accordingly." courtesy of Delta Kilo, on boards.ie
But i tend to believe that this is what is actually happening.