Casting with Gtk::VariantBase::cast_dynamic

[Povietkin Konstantin <povetkin k gmail com>]

What is the difference between casting of 
Glib::Variant< Glib::ustring >.get_type( ).get_string( ) = "s"
and 
Glib::Variant< Glib::ustring >.get_type( ).get_string( ) = "o"
In my mind both are strings.

When type is "s" Gtk::VariantBase::cast_dynamic does without error,
when type is "o" throws std::bad_cast.
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