Почему мое изображение выглядит более насыщенным в Windows, чем в El Capitan на том же откалиброванном мониторе?

Even if you are using the same monitor for different systems and that monitor is calibrated, each system can also have different monitor profiles. So that's one thing that can affect how your pictures look on OSX or Windows.

Another thing: the gamma of your system. By default, the gamma could be different on both system and it's possible the PC will always be darker than the default Mac one, it's usually the case. PC users usually have a 2.2 gamma and Mac 1.8. So if your goal is to look good ONLINE and 90% of your clients' computers are PC then you might want to adjust your gamma to 2.2 to have a better idea of what they see.

It's not uncommon for professionals to work with 2 monitor profiles; one for their print and one for web. In each profile you can adjust the gamma and white point (which should be D65.) This way if a picture needs to be re-calibrated for a specific audience online, you're more on target by using their monitor preferences than your "print" preferences. Since you are a photographer, that could be an option for you. That's what I do.

This link below gives more details and some tips about this. It should at least give you a starting point to research more on that topic:

http://www.kenstone.net/fcp_homepage/gamma_mac_pc.html

If you can check this for yourself, notice the green plants and skin tones are way more saturated in Windows than in El Capitan.

It's hard to verify this because we all see your website's pictures in a different way. The best would be to show 2 pictures of the same website on PC and Mac next to each others! Even the screenshots get corrected.

I'm on a Mac, the green plants do look saturated when I use my "pc monitor" profile so I guess this has something to do with your issue. There's more details, shadows and mid tones when viewing them on the Mac profile I use. That's why I don't think the software color profile is the culprit, but the gamma or monitor profile could be.

Just a note on this: Your pictures still look great to me, as an amateur and not a professional photographer! But it's totally understandable that you can see what's wrong with them and you're annoyed by this.

There is not a "calibrated monitor". What you calibrate is the "ecosystem" so you need to calibrate your computer + operating system + monitor (inclusive ambient light).

Take a look on a hardware to really calibrate them: http://www.colormunki.com/

I believe this is related to the problem concerning the "Ambient Light" correction embedded in El Capitan. This feature automatically adjusts the monitor's gamma levels based on ambient light - and well, at least for me the results are less than satisfactory.

.. in other words, it results in less-saturated images, washed out colors.

The fix: Disable "Automatic Brightness Adjustment" from the Mac Display options. This also disables automatic gamma correction.

Unfortunately, Apple hasn't released a "real" solution for this yet.

More information available here with comparison pictures

Unfortunately there isn't much you can do about that. Each monitor has it's own hue an saturation settings. For example older macs vs the newer retina displays.

From a working perspective always use the same machine to do your tweaking and editing so you know how it looks then save it with the proper settings for where the image will be viewed.