Have a question maybe someone can answer

[Tinydinomomma]

I have been raising domesticated mallard ducks for almost 4 years now. This year some of my girls have continued to lay even through the cold days (finally slowed down but haven't stopped completely yet). I incubate the eggs because they feel the need to share nests and when a few ducks hatch they are done and they leave the remaining eggs. So my question is, I collect eggs until I have enough to incubate, took me 3 weeks. They went into the incubator at the same time. When I candled them just before going into lockdown, they looked like they were at different stages of development. So I'm trying to figure out what causes that? Is it when they were layed or something during incubation? Because even after they went into lockdown, I had 14 total that hatched an it's like groups of 3-4 hatch then about 1.5 days later, I'll have another small group to hatch. Is there an answer or is it just part of incubating duck eggs and raising ducks? This is my first year with this wierd scenario. Thanks in advance.