It's a bug. Probably comes from the fact that Arc's variables are prepended by _ (for Arc2) or __ (Anarki).
arc> an-unknown-variable
Error: "reference to undefined identifier: __an-unknown-variable"
^
|
This one
Scheme-side 'ac converts quasiquotes by scanning for escaped expressions and converting symbols in those expressions to __, but apparently there's a bug here