Hello guys. Thanks for your answerS.
The "error" was simple, stupid. In fact the _manageOrderedTable is an instance of a class called ManageOrderedTable which inherits from QTableWidget.
In this class, there's a method called fillAll and I called setColumnCount() from there. Bad idea.
When I call this method from upon the for loop, everything is fine. I don't understand why.
I could understand if setColumnCount() were called too late, but in this case I should have a segfault, trying to call a cell that doesn't exist.
Well, I don't really understand why but now it works...
Tank you for your help, Patrick.