I think Gumby is really close to the mark when stating that most of the original airplanes are hitting, or have hit the "C" check cycle at or near the same time. Hopefully this will improve as we add more airplanes and reach certain economies of scale.
This short blurb was copied from an interview with DN in FastCompany:
FC: "Some costs are going to increase: The new planes will eventually need more maintenance, and employees will make more the longer they work here. How do you maintain your operating margins, or can you?
Neeleman: We've been having these conversations from the beginning. We're always looking for new technology that makes us more efficient or a better way of managing the business. If you look at last year compared to this year, fuel costs will be more this year, but that's true for everybody. And maintenance costs will be double what they were, because our engines have scheduled maintenance. The biggest chunk is for overhauls. We have 25 engines we'll send in this year and it'll be 30 next year. We haven't sent more than seven in previous years. But we've planned for this. We know we won't have a big jump the following years. Those costs will be predictable, and they'll be spread out over more planes. As we get bigger, we'll take advantage of economies of scale. We'll save on parts. We'll save on marketing."
So it looks like they went from 7 overhauls to 25 in one year. That's more than a 275% increase in overhauls.
I think it's still pretty remarkable, given the large increase in both fuel and maintenance, that costs went down a few % yoy.