I'm starting a new thread to discuss parametric adjustments and how they are most likely implemented in Photoshop and Lightroom.
In a Photoshop RGB document, every adjustment layer can be represented by three curves: one for R, one for G, and one for B. Curves, in turn, are splines, which can be represented by a table of floating point (x,y) pairs. This is why adjustment layers add so little to the size of a Photoshop document.
Furthermore, multiple adjustment layers can be reduced to a single set of R, G, and B curves. This is brilliantly demonstrated by Greg Apodaca's "adjustment layer inspector," which you can download here.
The inspector is just a PSD file, so there's nothing to install. Just open the file in Photoshop and watch the composite R, G, and B curves change as you modify the adjustment layers. Here's a screenshot of the composite curves when I've got a Brightness/Contrast, a Levels, a Curves, an Exposure, a Vibrance, a Hue/Saturation, a Color Balance and a Photo Filter in the Layers panel!
As to whether Photoshop implements adjustment layers this way, we will never know because the software is proprietary. For the same reason, we will never know if Lightroom implements its parametric adjustments this way. But the inspector illustrates that the mathematics is there to be taken advantage of. And I'm guessing that Adobe's developers are smart enough to take advantage of it.