Someone had to have been sitting on a few eggs. I have never owned ducks, but have been told they will start a clutch and leave them for a fairly long period of time the first few to several days until they have what they think is a full clutch, then start sitting full time - not unlike most birds when left to their own devices, so their eggs hatch close together.
I don't think that's the case here. I gather the eggs daily and any I'm not sure of the age get tossed. The ducks tend to lay all in one spot, but none have started setting yet, probably because I keep taking the eggs.
I found one of the girls sitting on a batch of like 20 eggs last year, too bad it was before I had the drake or I would of left her to do her thing.