The feeding schedule on the bag was developed for the commercial hybrid layers, those are the ones the research has been done on for laying flocks because those are the people that can spend the money on research. They control when those pullets start to lay by selective breeding, manipulating lights, and some adjustment to the feed. They want to slow the start of egg laying a bit so the first eggs are a little larger which means they are worth more, plus it avoids some issues with them starting to lay really young. Those pullets are bred to lay so well that if they didn’t slow them down a lot would start to ay really young. That’s part of why many of them cut back to a 15% protein developer for a while.
Our birds aren’t bred that way or managed that way. Unless you have a special need, like show birds that need to grow bigger so you need to feed a higher protein level, you have a lot of flexibility in how you feed them. Some of that depends on your goals. Another part of it is that if your birds forage for part of their food you’ve lost the ability to micromanage every bite. Or maybe you feed them treats so they eat something other than just the feed you buy. It’s not what is in just one bite, it’s how many total grams of the various nutrients they eat in a day, and even then it’s more of an average over several days. Protein and calcium are not the only nutrients involved in this either.
There are plenty of studies out there that prove that excess calcium can damage internal organs of growing chicks. Not all chicks are damaged and how much excess calcium they eat has a big effect. If yours forage or get a lot of low calcium treats where the purchased chicken feed is a small part of their overall diet, Layer won’t be nearly as harmful as when Layer with the excess calcium is all they eat.
All this can be really confusing. There are a lot of unknowns in this. The way I avoid all these issues is to never switch to Layer. I always feed a low calcium feed to all my flock, high protein Starter when I have small chicks with the flock and a medium percent protein Grower when they are older. I offer oyster shell on the side so the ones that need the extra calcium for egg shell development can get it while the ones that don’t need it won’t eat enough to harm themselves. The ones that need it seem to instinctively know that they need to eat it.