Clerics have access to the All, Astra, Charm, Combat, Creation, Divination, Guardian, Healing, Necromatic, Protection, Summoning, and Sun spheres (with minor access to Elemental). Druids have access to All, Animal, Elemental, Healing, Plant, and Weather spheres (minor access to Divination). The much better access to spells means the Cleric requires more experience to level up, the logic goes.
The high XP for 15th level Druids are likely RP reasonings.