The same can be true for AU fic though. Take for example the "X has a secret in their past/is secretly a..." type of fic. You can certainly write one that is very close to canon, like a secret that explains some behavior and fits both technically as well as in tone with the canon, or you can write something that might be possible with the canon but adds a type of angst unlikely to be seen in canon (secretly a hooker, horribly abused as a kid etc), or you can add something that technically may not contradict canon in any way, but is highly unlikely and not really in line with the universe (like someone is secretly a shapeshifting dragon and on the run from their people, therefore never shifting into dragon form also dragons hide from the public so the other characters don't know about them... or some scenario like that). Most people would label the last AU but not the first, but inbetween there is sliding scale where you could label it AU or you could say it's canon fic.