They are dropping all the time Fry Forever. That was my point. Your zero rate is even worse than my one in a year.
My all endler tank has hundreds of survivors every year. Each of my single species goodeid tanks has dozens of survivors each year. The Heterandria formosa produce nicely too. Even the female molly, that never has a survivor in the community tank, does this when she is separated and the fry do not need to deal with other predators.
It is only the community tank where I take no action that has low survival rates. The predation can make it so that you never see a single fry dropped. It was pure chance that even that one platy survived in the tank. She must have been a very resourceful fry.