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.
The most common way this happens is that, during the course of the day, the patron cancels their hold and then changes their mind and comes and picks it up anyway.