XGetWindowProperty() AnyPropertyType and XA_CARDINAL?

[Kai-Uwe Behrmann]

A atom is placed as a XA_CARDINAL type to the root window.

To get this atom XGetWindowProperty() is used.
AnyPropertyType fails and XA_CARDINAL succeeds in the req_type argument, 
which is the 7th one, to XGetWindowProperty(). Is there a reason for this?
The XGetWindowProperty manual says both should succeed.

kind regards
Kai-Uwe

PS: I like to answer a related question https://bugzilla.mozilla.org/show_bug.cgi?id=709732#c5