Let me write and answer for you, although this belongs more to computer graphics.se
This is not the answer you want its the answer you need. The mathematics behind color are very complicated. In fact nearly every system out there, every introductory class on color gets it terribly wrong. Mainly because they use a close enough approximation or an older definition on things and confusion of what the goal is. (it is my opinion that rocket science is easier because the goal is not as opaque)
The problem is that many of the explanations out there aren't wrong, they are kind of half truths. They get you halfway there by lying to you, which makes it hard to unlearn the things. Second is that our common language is incapable of conveying the needed complexity of the matter. We simply do not have a enough words for different aspects.
What is color? There are wildly different answers to this depending on who you ask and the definitions are not compatible. The talk about wavelength having a color that you were presented in a physics class is half right. Wavelengths do have color, but colors aren't wavelength. So the usefulness of this information is almost nil.
Ok so insert crude simplified ASCII graphics graph:
| Light Source |---->| Reflection |<-
+--------------+ / +--------------+ \ +------------+ +--------+ +-------+ +------------+
\ / +-->| Eye optics |->| Retina |-> | Brain |-> | Experience |
\ \ +--------------+ / +------------+ +--------+ +-------+ +------------+
\-----\->| Filter |
ASCII Image 1: Quite simplified model of how our visual system works.
So a graphic designer is interested in the Experience of color, while a color vendor might be interested in repeatability and thus of the part before eye optics. whatever the case following holds true:
- RGB values are not color definitions. hey are just signals to the hardware. Each individual hardware produces different color!
- SO without specifying that your system is calibrated/Emulating standard XXX YYY it is unknown what the color actually is.
- If the device has not actually been calibrated recently then we don't know what signal reaches users eyes.
- the Brain is exceptionally complicated.
- Just the physical structure of retina does some color/shape processing!
- The white balance in a human brain is tricky to understand etc...
Ok now we are ready to understand the question a bit better. Why is HSB or HSL different? Well sometimes they are and sometimes they aren't? This entirely depends on how you calculate lightness and brightness.
Lightness is a bit controversial, how do you calculate it? Well there is several different calculations you could make. You could just average the R,G and B values. But then if you do that then you'd have people complaining that your blues are unnaturally of different lightness/brightness than your yellows and reds.
Simply the differences stem from how much mental work each individual implementer was putting in. So HSL and HSB dont have definitions that is just one formula, and the formula depends on.
- What the developer was trying to model
- What kind of hardware the developers happened to have at the time
- And whether or not they implemented calculation in integer arithmetic or not.
WTF? Well, there is no scientific motivation in turning color into polar coordinates. Its just considered more elegant of a shape than that and artists have done the transformation because it suits them in the past so there that.