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It could be more then one factor. An incubator, no matter what brand, has hot and cool spots and the eggs devoloping in a cool spot will devolop slower, hot spot faster.
If using a Sportsman, new eggs go in the top tray. second week they move down one slot and so forth. Also when candling I swap my eggs and change their position in the rack paying attention to the fullness of the eggs. If fuller they are devoloping faster and need to be put in a place where the slower eggs were.
Another trick has to do with storage of eggs. If storing eggs to get a full tray store them in the fridge, as long as they don't freeze they will be fine. Warm them to room temp before adding to the bator or else they may sweat and introduce bacteria to the eggs. An egg starts to devolop at around 80 deg. and keeping them in the fridge will put them in suspended animation till it is time to set them.