There are two signals emitted if the current item of a combobox changes, currentIndexChanged() and activated(). currentIndexChanged() is always emitted regardless if the change was done programmatically or by user interaction, while activated() is only emitted when the change is caused by user interaction. The highlighted() signal is emitted when the user highlights an item in the combobox popup list.
In other words:
activated() is sent when you make a new selection.

You might have a selection in the combox, but focus on other widget.
When the focus returns to the combox, the current selected item will be highlighted, and the highlighted signal is sent - in this case, no selection change is made.
Use the one you thik is apropriate.