"Maybe sometimes it's: hatch-rate." <---- You are right about this. Usually after shrimp is berried for 2 weeks, you will see a significant number of eggs are reduced. The shrimp will dump those eggs that she think it is not hatching.
Other than this, there is another important stage. The first week after hatch is the critical period. During the first week, they will have their first moult. Once they successfully moulted, they more or less will survive. This is the observation you have about "the number of shrimplets I see after 5 days, that's about 95% of the shrimp that will grow up". I'm not sure what contribute to the survival of first moult.
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