Manual brightness can be set at 100% which is 25% less than the maximal brightness the phone can attain. The 25% brightness is only accessible by the system when auto brightness is set and there is very high ambient light, which is completely understandable. However, if the engineers were smart then manual brightness being set to 100% would TURN ON auto-brightness (not known but hidden) until the user adjusts the brightness below 100%. When the manual brightness is set to 100% and auto brightness turns on, it will use the ambient light sensor to judge whether it should adjust the brightness into the 25% off limits zone. When the ambient light falls back down, it adjusts back to a minimum of 100% or it turns off if the user adjusts the brightness below 100%.Or maybe you'll re-phrase it in plain english.
I can't describe it any simpler.