I wanted Ameraucana and Black Copper Marans when I first started with chickens.
Faced an early disappointment of those breeds being widely misrepresented,
and hard and expensive to find real and good ones.
So I had to wait a few years and finally found some local BCM eggs(not great lines) and...
Would be hard to actually quantify this, wonder if what you read came from a study or is just anecdotal supposition. Has someone actually hatched a bunch of pullet and hen eggs from the same parentage then tracked them living in the exact same environment thru their whole lives to see...