Or maybe there's a bug somewhere? The default assumption should not be "pg is an elitist jerk," but "pg is a fallible human being." Whether or not pg is an elitist jerk is irrelevant, as the answer is probably that there's a bug/some unexpected behaviour hiding somewhere in arc.arc or ac.scm. In fact, I think that is the case, and the problem has to do with translating lists between Arc and mzscheme: see http://arclanguage.org/item?id=6993. But it's always best to analyze first, since you get more done that way.
: In fact, "so-and-so is a fallible human being" is almost always the best first stop: Hanlon's Razor ("Never attribute to malice that which is adequately explained by stupidity.") and all that.