The numbers on the dashboard are from Amazon's "order" report and show all sales as soon as the customer ordered something. The numbers in the monthly overview, on the other hand, only show completed transactions after the payment went through. That's why you might see an order made in one month only in the following month's overview - because that's when the money came in. On top of that, remissions or adjustments (e.g. Amazon broke something and replaced it) are not shown in the dashboard.