What is DefaultOutlineCellRenderer? Is it the netbeans class?
If so your problem could be because this class adds a border to indent the text an amount. If you want to provide all your own settings for the returned JLabel then rather than calling super.getTableCellRendererComponent(..) just create a JLabel, configure as you require and return that.
To save unnecessarily creating loads of new JLabels it would be better to create 1 JLabel, store it in an instance variable and reuse it.
subject: NB Platform - Vertical align in OutlineView