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Last names (and even first names) get switched up a lot in South Park. The name game is just a side effect of the way the episodes are made. When you're creating a show, start-to-finish in 6 days, most of the time goes into getting the episode finished and on the air before the deadline. And some little details slip through the cracks. So Harris becomes Donavan, Swanson becomes Valmer, and Williams becomes Black, and Mr. Garrison becomes a woman who becomes a lesbian who becomes a man again thanks to a penis mouse. It's all pretty simple.