According to documentation:
https://doc.qt.io/qt-5/qtqml-typesystem-basictypes.html
https://doc.qt.io/qt-6/qtqml-typesystem-basictypes.html

There is:
> double Number with a decimal point, stored in double precision

So "double" is valid QML type, but wtih qtbase(659817e287c0948829b8d3c4239cc7fa6fcf70a2) and qtdeclarative(2181ef9d29c71273accab710a68907c5d42771db) in qml/builtins.qmltypes I got:

 Component {
        name: "double"
        prototype: "number"
        exports: ["QML/real 1.0"]
        exportMetaObjectRevisions: [256]
        accessSemantics: "value"
    }

c++ double mapped only to "QML/real".
I suppose it should be mapped into both "QML/real" and "QML/double",
because of "double" also valid QML type.