That is interesting -- are the ones that start at different times from different parent stock? Just curious.
When we think about it, it is more likely that there is going to be variation.
Here's a funny story -- we did an Artificial Insemination (AI that doesn't mean Avian Influenza) project on our registered cattle about 10-years ago. Every cow and heifer was inseminated the same day -- that had come into heat in the previous hours -- and we were kind of naive and thought that subsequently they would all calve on exactly the same day. Like one day -- there would be 15 newborn calves - then the next day there are 10-more --- or however many cows we had inseminated in each session according to the gestation period of the cows. -- They spread out the calving over a couple of weeks. That's mother nature for you.
Animals don't read the books!