With perfect execution you can get 1.8 villagers ahead with chinese (dropping of food in TC at the perfect time required). These vills also work permanently for almost the entirety of the game, collecting a lot of ressources. And then ofc the mentioned Ressource savings in the techs.
Ah and Chinese Farms have more food and Their TCs have more garrisons space aswell.
They have a lot of little factors that add up.
As you said Britons already start with a slight advantage (equivalent to about 100 W). But they have also cheaper TCs which allows them to pull ahead later even more.
Especially for the britons there is one factor worth mentuioning again (I already did previously).
For Birtons my calcs say it’s optimal to make the TCs with only 2 villagers, but earlier. This allows you to make the Farms needed to supply the TCs while the TCs are going up and not partially before. And it also saves you villager time, whcih can just be used to collect ressources. (Also less wood cost on the TC means less floating ressources with optimal macro).
It’s true that we rarely see he Britons bonusses used that way. But if I evalueate it from a mathematical approach I have to assume a optimal booming macro. I can’t assume not utiilizing the bonusses to their full potential.
This is also the reason why Sicilians are good-ish in this comparison. The Farm Bonus isn’t that strong, it’s actually their TC Bonus that benefits them the most. Like Britons Sicilians should only build their TCs with 2 Vills.