@U_Stormcrow understands the math on this better than I do.
Unlike commercial layers, backyard birds generally have free access to plenty of lower-protein options such as pasture.
While your point about amino acid balance is important, this statement is not accurate.
Layer feed is designed to be the most cost-effective means of supporting high-production, commercial-strain layers through their initial productive period up to their first molt, at which time they are...
Here's how to get an actual body condition score: http://www.poultrydvm.com/featured-infographic/chicken-body-condition-score-chart
Layer breeds are often "skinny" compared to meat breeds.