Absolutely do not pay attention to dates listed.
Hendersons does not give a date for POL but does give guidelines for when they Mature. For example, Leghorns says extremely early maturing. They will lay eggs as early as 16 weeks. Then Brahmas says Slow Maturing so they will reach POL after the average for all chickens which is 22 weeks. Very slow maturing would be in the 30 week range.
The descriptions give you a general idea for how long it will take. Like I posted above, it also depends on the time of year.
I get you Ron. That might help someone get an idea what they are looking for when they are starting out.
I have seen hatchery type Rocks start laying at 20wks, and "Heritage" Rocks start at 32 wks. That is almost a three month difference.
I have seen hatchery Reds start at 22 wks, and "heritage" Reds start @ 28 wks. A month and a half difference.
I have had a decent strain of NHs start at an average off 22 wks, and anther good strain start in the 26wk range.
My Catalanas start in the 20wk range. About typical for a Mediterranean breed. Just too much variation from one strain to another.
Then add on the when they hatch, like you said, and management and environmental differences.
I think it is a matter of getting to know your strain. You know when they are hatched, management, and conditions. If you think they need improvement, put pressure on it in the following generations. If you are happy with where they are at, do not worry about it.
These are just opinions that I have.
It is also my opinion that as we breed to the standard we remain mindful of these things. Keeping in mind the role these birds had historically.