ADL is a bit of a black box - Nikon does disclose the specific exposure adjustment in most manuals.
For best results with ADL, you need to be using Matrix Metering.
ADL applies a curve in processing the image. The curve is in place of any in camera settings for Brightness and Contrast according to the D300 manual. So if your Picture Control selection is one that applies low contrast or low brightness - like Neutral - you could end up with an increase in contrast and brightness using ADL.
ADL still may require exposure compensation or other adjustments.
I'd encourage you to do some controlled testing and share the results with us here. Because ADL is a bit of a black box, we are all trying to learn more about exactly what it is doing and where it performs best.