What causes differences in air cell sizes?

[WhiteTreeOfGondor]

Does anyone know why some of my duck eggs might have a smaller air cell than others? They are getting ready to hatch and most look good but a few look too small. There are some internal pips so I raised humidity, but I'm worried about those that are still small. And one egg is pretty porous (I know you aren't supposed to incubate those but I gave it a chance) and the air cell is huge. It's taking up half the egg. Could that be a problem for the duckling like small air cells are?

 

[Yardmom]

The porous eggs may have been loosing more moisture. Also the air cells draw down just before hatch.

 

[WhiteTreeOfGondor]

The porous eggs may have been loosing more moisture. Also the air cells draw down just before hatch.

Thank you. Only one egg is noticeably more porous than the others, but maybe there are differences in the others that are harder to see. Drawing down is a good point too. With my first hatches all the eggs were more or less at the same stage at the same time, but this is the largest hatch I've done so far so maybe there's some that aren't as ready.