The clarification is simple.
One derating is called an adjustment (fill) and the other a correction (temperature). You must always apply both.
Sometimes one or the other will give a factor of 1 and can be skipped, but never ignored.
In the case of roof exposure of a raceway, there will be one temp correction but it will be based on the outside ambient plus the roof adder.
I don't know I like the fill term you used. IMO both are adjustments, and both are items that need consideration.
If we have a conductor rated for X amount of amps the idea is to prevent overheating conditions of the insulation. The conductor itself will operate much higher temp then the insulation before damage occurs, so it is essentially the insulation we are protecting when determining conductor ampacity under art 310.
When a conductor carries current it produces heat, maybe not a lot of heat but enclose it in a raceway or cable and that heat is trapped in there compared to free air applications - that is why ampacity tables exist for free air applications as well. Sometime somebody has determined the effects start to increase enough with more then three current carrying conductors that we need to make adjustments to the overall ampacity, and additional increases at other current carrying conductor levels to protect the insulation from overheating. These results maybe are not perfect but were agreed on as a somewhat simpler basis that works across many conditions then the complexity of determining every possible condition and getting exact results.
Additional ambient temperature does naturally increase the conductor temperature and also needs accounted for. Keep in mind low ambient temps do allow you to increase the final conductor ampacity instead of decrease it.
