But there are just two of us that are 18th.
I was running Acute Delirium with my 18th Wiz and a 18th Cleric. We had a 16th level Hireling and a 17th Level Hireling plus the Onyx panther. When we finished the quest I saw that it seemed really low. Popping open the XP panel showed a -25% penalty because a 20th level was in the group. However, there certainly wasn't.
We did not leave, or have another group member come and go. It was just the two of us plus hirelings. In and done.
Has anyone else seen this? Is it a fluke? A bug with a hireling? (I did bug this in the quest.) A handicap because my daughter is so uber? Whatever it is, it is not cool.