"Increase Indent" and "Decrease Indent" buttons in the Ribbon (the same commands are invoked when Tab is pressed in specific situations) apply different indent step to the selected paragraphs, depending on the number of selected paragraphs: 
- when applied on one paragraph, the indentation is changed with 18.0 px. 
- when applied on two paragraphs, the indentation is changed with 36.0 px.
- and so on.