Short Answer: Because a patron checked the item out after the hold was cancelled or removed or expired. This is perfectly normal.
Long Answer: The Clean Hold list will list all items with INACTIVE holds and print out the items' Current Location. Holds become inactive for a variety of reasons--none of which preclude the items from getting checked out. The reverse is also true--just because an item on the list is currently CHECKEDOUT, doesn't mean the item can't also have an INACTIVE hold on it.