Dan,
It is actually the 'steppedArea' series that is causing the extra space around each domain value, unlike the 'area' series that behaves more like 'line' series. If you change 'steppedArea' to 'line', you'll see that the extra space disappears. The area and line series wouldn't know where to draw in that extra space around the values because there is no next value to connect to, whereas the steppedArea series makes its transitions half-way between domain values, at least when the domain values are discrete, as is the case with your string values for the months. But if you change the domain values to numbers instead, then the steppedArea series behaves more like line series.
Hope that helps explain what you are seeing.