Strange development rates

[Maple128]

So i've noticed in my newst batch of eggs the developments seem to be strange. Most of them look to be around the same stage but one paticular egg already has gills and a pronounced tail, any reason why one would be so ahead of the others? they're currently around 5-8 days old.
will upload a photo soon

 

[Maple128]

also, would this mean the other eggs arnt developing? or much slower than the paticuarly fast one? also why would some of them be much larger than the others?