The lowest brightness level is pretty dim and shows 0% on the overlay brightness bar while adjusting, but a "lower" setting is somehow allowed through and it sets the actual brightness to 100% (the bar stays at 0%).
Adjusting the brightness up from this setting takes the bar to non-0% and to the 2nd actual lowest brightness level -- in other words, 2 "steps" are allowed down, but only 1 "step" up.