Pure primary color shows other primaries?

Question

Hi, I'm trying to get my head around exactly how the HSL sliders in Lightroom 4.4 control Saturation and Luminance. In the process of doing that, I created 4 PSDs in Photoshop, each with a single layer (i.e. the background): of red (255,0,0), blue (0,0,255), green (0,255,0), and a gray image (127, 127, 127). When I open the PSDs within Lightroom, they show in the histogram as having a sizeable contribution from the two missing primaries in each: if I open the blue PSD it shows Red 41.1%, Green 18.3%, and Blue at 93.8%, if I open the red PDS it shows Red 75.4%, Green 34.7%, and Blue at 13.7%, if I open the green PSD it shows Red 61.0%, Green 94.2%, and Blue at 37.8%. Can somebody please explain what's going on here? I'm not saying this is how I intend to use Lightroom, I just created these test images so I could see how the extremes of Saturation and Luminance adjustments would affect the colors and the results I got surprised me.
First, some questions about Saturation.
Let us say that I have opened the blue PSD document. The Histogram initially shows Red 41.1%, Green 18.3%, and Blue 93.8%  If I drag the Blue Saturation slider all the way to the right (i.e. +100), the color on the screen does not change, but the Histogram changes and now reflects Red 39.0%, Green 10.8%, and Blue 93.8%. The color looks identical, but the Histogram is now different. What is happening here?
Also, let us say that I have opened the blue PSD document. I would expect that if I grabbed the Saturation Targeted Adjustment Tool and dragged up or down, that only the Blue slider would change, but instead, I see both the Blue and the Purple Saturation sliders changing. Why is this?
I get similar results regardless of which primary RGB color PSD I open.
The one behavior I witnessed within Lightroom that I expected to see was that if I dragged the Blue Saturation slider to -100, the image would become gray.
Now, on to Luminance.
Is Luminance synonymous with Brightness in Photoshop? Analogous?
As with the Saturation Targeted Adjustment Tool, when I use the Luminance Targeted Adjustment Tool, instead of seeing just the blue Luminance change, I see the Blue and Purple Luminance sliders change. Why is that?
If I drag the Blue Luminance slider to +100, the color becomes somewhat like a pastel, i.e. as though white were added to the color.
Ultimately, I have no idea what Luminance is really doing in Lightroom.
Any and all help is appreciated, thanks!
Matt

Answer 1

Thanks for your very thoughtful and detailed answer!
"First, it should be clear that saturation is not absolute, it is relative to color space. A 255/0/0 red in sRGB is not the same as a 255/0/0 red in Adobe RGB. The numbers are the same, but the color is different. Inversely, the same color will have different numbers in sRGB and Adobe RGB."
OK, I see this. Both sRGB and Adobe RGB are color spaces that are derived from the RGB color model and they each encompass a different gamut. Now, your screenshots got me thinking that I had been sloppy in creating my test documents because I had simply accepted the default color profile (i.e. for me sRGB). So, I went ahead and created two new PSD documents: one which used the ProPhoto profile, and another which used the sRGB profile. My results were even more confusing!
The Red PSD with a ProPhoto Profile
The first image I created in Photoshop and examined within Lightroom had the following profile according to the File Info dialog within Photoshop CS6:
When I opened that image within Lightroom, the histogram showed zip/nada within the Library Module:
and showed the following within the Develop Module:
Now, I do understand that there is a difference between the two histograms in general, and that it is generally understood that the Develop Module shows a more accurate histogram, but to show nothing at all? I don't understand that. So:
Why does the Library Module's histogram show nothing at all?
As can be seen, for a primary color image created with the ProPhoto profile, it shows 100% of the primary color, which is what we would expect, correct?
However, I don't understand the two spikes at the extreme left and right. I guess I would expect to see only a single spike at the right?
The Red PSD with a sRGB Profile
The second image I created in Photoshop and examined within Lightroom had the following profile according to the File Info dialog within Photoshop CS6:
When I opened the image within Lightroom, the histogram showed the following within the Library Module:
and showed the following within the Develop Module:
As can be seen, the histograms match up perfectly. Now, as to the values displayed:
Even though the image is 'pure' red (i.e. 255,0,0 in sRGB) am I to understand that it is displayed in the histogram as 75.4,34.7,13.7 because within the color space being utilized by Lightroom, that sRGB (255,0,00) would map to a point in the Lightroom color space that is only 75.4% pure red, is 34.7% green, and 13.7% blue?
Because the 'Red ProPhoto' psd from the first example showed as 100% red in the Develop Module's histogram, is it safe to conclude that the 'working' color space within Lightroom is akin to ProPhoto RGB?
Thanks to all,
Matt