Now you have a new problem of no one agreeing on a standard saturation level. I'm noticing this more with "dark themes": everyone has a different idea of dark gray.
Since dark themes naturally lead to subduing all other colors except for little accent pieces, the screen is a collage of dark grays to the point that, instead of the background fading away, the background is now stealing the show from everything else. It's this dismal bouquet of gloomy colors and it's hard to focus on the content.
Contrast this (pun!) with terminals and console apps: your terminal is a constant background color while you're working. It doesn't change its shade of gray as you go so it really does stay as a "background" and isn't distracting.
Finally, I feel dark themes exhibit this more than light themes. I feel years of different subtle paper whites has made me less sensitive to variations.
* People have monitors with different gamma curves, and even if you try to match them slight different output for top white and bottom black. Heck, on bad monitors these things can vary across the screen or if you aren't sitting perfectly head-on.
* People are working in different environments so ambient like that they are seeing the screen against (home & office lighting, light from windows, reflections of those off home/office decor, and so forth) will vary.
* Peoples eyes vary in a considerable number of ways, even ignoring those with variations significant enough to be considered “defects” from the norm.
* These things can all vary over time, over different cycle times.
It doesn't just affect blacks and grays: on a simple dashboard I've created pastel colour backgrounds are used as subtle highlights (used to separate things, guide the eye a little, to indicate status of things (though there are other, less subtle, indicators when this is significant), and to just make it look nicer (as coldly objective as I can sometimes be, even I appreciate a bit of effort there)) that are useful (you miss them if they go) but don't want to be attention grabbing. I've needed to tweak the colours chosen because while they did the job on my screens they were too similar or too almost-not-there-at-all on other people's.
I think the variations are intentional. If I were a designer and looking to match backgrounds, I wouldn't eyeball it. I'd screenshot the target and eyedropper the color - that's independent of panel differences. But maybe I'm old fashioned and not a designer anyway.
"Dark themes" are the new sexy right now and everyone's having a play at making their design palette stand out. I think good usability is taking a back seat to artistic freedom and expression.
And don't forget about the most obvious factor, which is that people simply want their website to look a certain way and don't want all websites to look the same.
Since dark themes naturally lead to subduing all other colors except for little accent pieces, the screen is a collage of dark grays to the point that, instead of the background fading away, the background is now stealing the show from everything else. It's this dismal bouquet of gloomy colors and it's hard to focus on the content.
Contrast this (pun!) with terminals and console apps: your terminal is a constant background color while you're working. It doesn't change its shade of gray as you go so it really does stay as a "background" and isn't distracting.
Finally, I feel dark themes exhibit this more than light themes. I feel years of different subtle paper whites has made me less sensitive to variations.