Question: When is a ColdFusion structure not a structure, as checked by isStruct() ?
Answer: When the structure is a CFC !

In ColdFusion, isStruct() returns true for structures and CFCs. If you really want only structures, you need to add 'not isObject()' too.
Brian Kotek confirms that "CFCs can also be treated as structures for most purposes .... StructKeyExists() [etc]".