clarification on GtkAlignment

[Mark Ivey <zovirl1_list sbcglobal net>]

I don't understand something about alignments, hopefully someone can
help me out.  My window contains a 4-row vbox filled with alignments. 
Each alignment has the same settings:
        x align: 1.0
        y align: 0.0
        x scale: 0.5
        y scale: 1.0
Each alignment also contains a 1 button.  The buttons have different
labels, like this:
        a
        as
        asd
        asdf

Now, I would expect all the buttons to take exactly half the window, but
they don't.  The buttons with shorter labels are slightly wider.  FWIW,
this happens with labels also, not just buttons

So what's the deal?  

My ultimate goal is to have a layout like this, where all the widgets
are lined up 1/3 of the way out from the left edge:

Section heading
----------------
     widget
     widget
     widget

Another section
----------------
     widget
     widget


...is there a better way to do it?

-Mark Ivey-