As the eggs age, they tend to take on water and get larger, and I think there is a fine balance between temps, time, humidity and development. When the eggs are laid, they're obviously fairly hard. As time goes on, they soak up humidity and moisture from the environment, expand, the shells soften, and then when the time comes, it's easier for the gecko to breach the interior lining and hatch. The more that time goes on, generally, the more moisture eggs take on and at least in my experience, sometimes they can fail before the embryo is fully mature.