When you log into your dashboard, on the first page, it shows you your sales for the past 7 days. That means it can change from day to day as the month progresses.
In order to see your total sales for the month, you'll need to run a conversion report.
You'll find conversion reports under the Reports menu in the sidebar.
You can change the timeframe from past 7 days to any time range, or to a particular time range such as this month, last month, etc.