Quick Exploration exp reward is variable depending on basically the thing(s) they return, as in higher level/rarity item(s) collected = higher exp.
As the ability to have a higher percentage of getting higher grade item(s) is directly tied to the IL of the retainer's gear, and secondarily based off the primary retainer job stat (which then allows access to higher payout percentage explorations), improving your retainer's gear will greatly increase the chance of higher exp exploration runs.