Interestingly, on my early 2015 13" MacBook Pro, the very lowest brightness setting seems to have no change in brightness on those last three subdivisions. Once I get down to one full "block" of brightness, option-shift-F1 moves the slider down in quarter-block increments but the screen gets no (perceptibly) dimmer until it shuts off entirely. All the other brightness settings have perceptible changes in brightness at every subdivision level.