If the object has a length property, it is assumed to be Array-like. There are several Objects that have Array-like behavior, like jQuery objects, DOM NodeList objects, and Function.arguments.
In this case, you've specified a deep copy so when $.extend sees the nested object in prop it thinks the object is Array-like. I've opened a ticket on this because it may need to be revisited: http://dev.jquery.com/ticket/4227
