Everybody has the same XP Cap Nazgul, so I think you are mistaken.
Maybe you are referring to the fact that druids can only level up to level 15, whereas other classes can reach higher levels (I think the bard and thief classes can reach the maximum of 23rd level). This may have something to do with the druid hierarchy in AD&D. A druid cannot proceed above a certain level unless he/she challenges a druid above him in the order.
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Melusine, High Queen of Fluffies, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia