. I'm no color expert, but the details of the calculation would be of interest since delta E includes luminance, chroma, and hue...
Given two values in CIEL*a*b*, L1, a1, b1, and L2, a2, b2, the CIEL*a*b* Delta-E is:
sqrt((L1-L2)^2 + (a1-a2)^2 + (b1-b2)^2)
Stated in words, it is the distance in three-space between the two values plotted in Cartesian coordinates.
It is well documented that ACR does increase saturation as exposure is increased and this is by design, as it is preferred by many users. However, a shift in hue is unwelcome by most.
There is no measure of saturation in CIEL*a*b*. There are good technical reasons for this, which I won't go into unless requested to do so. There is one in CIEL*u*v*, but Photoshop doesn't support that system, and most photographers aren't familiar with it. Some have suggested that a saturation measure for Lab might look like this (dropping the stars):
PseudoSat = Sqrt(a^2 + b^2) / L
My tests indicated that Lightroom does not increase this quantity upon increasing exposure. It does -- and should -- increase chroma, defined as:
Chroma = Sqrt(a^2 + b^2)
If you imagine CIELab in cylindrical coordinates instead of Cartesian ones, luminance goes up and down, hue is the angle, and chroma is the radius.
If you look at the actual color plots, you can see that there is no general chroma or hue shift, just shifts that affect both.
Eric has put my testing methodology into question, and I'm going to have to fall back and regroup.