Before Android 9, Android device manufacturers would use recommendations from the screen manufacturers to determine how bright a screen should be based on ambient lighting. The baseline mapping was the same for all users, and phone owners would have to constantly use the brightness slider to adjust the display to their liking. Setting the slider to the left of center made the screen dimmer than the preset, and moving it to the right of center made it brighter than the preset. Messing with the slider would force the user to constantly make adjustments each time the ambient settings changed.
Google says that the model it uses for the improved Adaptive Brightness can be updated, and will be fine tuned based on real time usage. Pixel models have this available now, and the company is talking to Android manufacturers about adding the improvement into the Android Pie builds for their handsets.