Photoshop color picker question

This is probably "gray component replacement" (GCR). It's built into the profile, and the aim is to use less ink (or cheaper black ink) to produce the same color.

You'll find that the RGB values are identical. It's the same color.

If you change the Sample Size of your Eyedropper tool you may see even more variances.

Yes, you should see differences between lighter and darker colors. The amount of "black" that is in the color has a dramatic effect. CMYK includes black RGB does not and so PS has to compensate for that.

I can't give you an absolute solution but essentially you are defining a color using one color method and your document uses another. Your document is RGB so when you mix a color using CMYK there are going to be inconsistencies because PS is having to translate your color to the color mode of the document then apply it. How PS does this is dependent upon your color profile and conversion settings.

When I try to recreate your scenario as you describe I get different results with only a slight change in color numbers. I know that this is due to the fact that your color profile settings differ from mine. I have turned off my Use Black Point Compensation in my document color settings. You will want to be careful when adjusting these settings as they can have a major impact on your color management workflow.

I hope that this helps.

Doesn't the answer lie with the CMYK working space chosen? (FOGRA39 is different from, say, U.S. Web Coated (SWOP) v2.) I don't know for sure as I'm not a propellerhead on this matter.