Let me say with all due respect and having heard this before....
Hogwash....
The sex is determined by the hen, some hens produce more boys some more girls..
Just lke some men have all sons and some have all daughters... Period...
Sorry to be blunt, (well not really) but this non-science where some one says test show, or studies prove....Never have a trustworthy study to quote just some anonymous test or study...
I need a University study or government study for me to buy it...OR I put it in the same class as dangling a ring over a chick to determine it's sex...
Maybe a ring dangled over a egg would work...If it swings left we keep the egg, swings right we eat it.....
I just listed them all... mostly for fun.
I think the hen chooses as that is the basic biology of the chicken and like you I think some just are more prone to produce W or Z. But people say that is hogwash too and if you are not getting 50/50 we'll you just have not hatched enough eggs, it's coincidence etc, but science actually has discovered in mammals there is a reason some horses sire more girls than boys, so I am inclined to think it is the same reason in hens. So then you got to do some serious mommy egg sorting to figure out which birds produce more boys or girls, then try to pick Roos from hens that produce the gender you want at the higher ratio and use only the hens that produce the gender you want at the higher ratio.
I am on the fence on outside influences but I know I have read conflicting studies... so then you got to figure out why the data conflicts, sometimes methods were different, goal of study was not really that subject but was a side finding and so on.
The crazier stuff is just fun trivia, or like I say organizes your farm chores nicely.
But sometimes some old weird wives tale proves true...
Egg shape is one that is surprising me, not saying it is confirmed just prelimerly studies are starting to happen with some results that show it maybe possible. The egg thing maybe be like wing sexing, works on some breeds but not others, some people are good at it and others not. I don't know as there are other wives tales tied to egg shape that have nothing to do with sex but how the chick will develop too.
Myself if I was breeding and the ratio one way or the other was consistently skewed in a direction I disliked I would start trying to isolate hens to collect and mark eggs to track who was producing what.. spreadsheet time, raw data collection time... despite my love of old folklore I am also pro scientific method and putting science to work.