Originally Posted by
Lerend
Correct, the quest rewards scale negatively with the level of your class when you turn them in. This is to prevent you from doing a level 45 quest on your high level LNC, swapping to your level 2 CNJ, turning it in for 50k exp and immediately shooting up to 15 CNJ. If the quest is, for example, level 10, as long as you are above level 10 as your current class when you turn it in, you will get full exp. Some quests let you pick them up before their "optimal" level requirement, and if you complete them before that level you will also get the reduced exp rate like you mentioned.
I've noticed some quests have ~1000 XP (or whatever the value is) directly listed in the journal entry to denote this discrepancy, but some still list a flat rate.