The trouble with these simple charts is that they rely on the latitude of the film to fudge the issue. If you look at the (?) Victorian grand father of the "Sunny 16" rule you'll find it talks about bright sunlight in June. And June's daylight is not the same as December's, as we all might just have noticed.
The rule then was to use the reciprocal of the H&D speed at f/8. I'd like to track the papers down but it's complicated. In October 1937 a magazine published an article about H&D speeds and a reader wrote in quoting a RPS paper of 1920 that was a reprint of something else but he (the reader) quoted the (?) RPS's "Sunny f/8" rule and that's all I know.
FWIW you can use the sunny 16 rule in May, June and July and amend it to sunny f8 in November, December and January. The rest of the time use f/11 but that's just my version of it but it's based on everything I've seen about exposure without a meter including versions by Zeiss, Leica, Kodak, FED, Welcome, Johnson's and so on. This is not a popular opinion, btw...
Regards, David