Lightrooms' sliders are weighted distributions. It is not as simple as "Whites" are 127-255, "Blacks" are 0-127. The distributions are weighted, and different sets of sliders affect different ranges in different ways. Whites affects a broad range, but obviously affects brighter tones with more weight than darker tones. Same with blacks, only it's the inverse. Highlights and shadows are much tighter weightings, however they still affect a fairly broad range of tones.
Exposure is not midtones...it is the entire range of tones, highlights to blacks. Exposure is weighted, however, as the general idea with the exposure slider is to simulate what happens when you actually change exposure in the camera. It's just weighted less than the other sliders.
The tricky thing about these sliders is, as you move them around, more tones in the image fall into the ranges they affect, so there is a bit of a dynamic shift occurring as you move the sliders around. That's why shifting highlights to -35 may often do a lot, but shifting it to -75 does less, and shifting it to -100 does even less.