In documentation (http://qt-project.org/doc/qt-5/qml-qtquick-controls-combobox.html#activated-signal)

activated(int index)
"This signal is similar to currentIndex changed, but will only be emitted if the combo box index was changed by the user and not when set programatically."

This is not quite true. When CurrentIndexChanged() signal arrives, the index is already changed and you can set new index in that signal (currentIndex = 0 for example)
However, in activated() signal, the index is not yet set, so setting currentIndex at that point will not change current index. This would be good to be documented.