Challenges and XP

[Valakai]

Ran a few challenges solo. Starting to actually like some of them. What I was curious was how does the xp scale. I have so far only ran them at lvl10 but the xp seems wonky. It always says something like 1400xp. 1st time I completed was pleasantly surprised to get 5k xp. Next time about 2000xp. Another challenge 900 xp. Then 4000 on one of them. I'm sure there is some very smart and clear logic behind it all. Been mostly 1-2 stars with one 3 run comp. If somebody knows how it scales then I would appreciate the info. Felt kind of cheated when I got 900xp for completed run.

So is xp tied to lvl of the quest vs char level?

Is it based on the amount of stars?

Does it decrease the more times you complete?

Thanks in advance.

[Terminus-Est]

Quest vs Character level: Yes.
Stars: Yes; you get xp for optionals and additional one-time bonuses for tiers of optional completion that can stack in the same instance; one at 3 stars, another 4 and assuredly one at 5 (I haven't had any 5-star runs yet)
Decrease: Once you get the one-time bonus (which are quite large) you cannot get it for that tier of completion again, so kinda.