If it's not working as intended, then the proper post would be in the forum's bug report board. However, a great deal of the variability can be external factors, Like Free Company Buffs, Rested EX, Food, Armory Buff, "Path to" buffs and so on. This is as someone who almost religiously uses the trust system for Exp of multiple jobs and have done both 71-79 and 81-89 dungeons.

However this was a topic of discussion in our group not long ago and we determined we agreed with the current format is intended. The capped dungeons are for Tomestone grind as well as whatever is available in those dungeons, like Minions, etc. However the Leveling dungeons are clearly marked for assistance in leveling as they also provide guaranteed gear pieces at the end of each run that are to assist the facilitation of leving through the dungeons. You can appeal for a change if you like, but I don't expect this to change nor am I upset at this fact. At 80, you're running them for story, poetics, and individual story drops. The story alone should get you through leveling your primary job to 81, and Poetics alone will get you over the hump gearing needs.

If you're having problem with poetics, I don't know what to tell you - maybe play any portion of the game past level 50 content as they flood poetics at people.

Commentary as to the effectiveness of each dungeon for each job would have to get very, very specific to verify that there's an issue here.