+0
You sold an apple on September 30. But the sale of this apple will be included in the total sum of October because of your totaling job. While you're totaling/summing the sales in September, the apple's sale occurs. This kind of thing has you include it in the sales in the following month.

You say,
Sales are usually summed up in the right month accordingly, but some sales are included in the following month.

Does this make sense? 
+1
Sales figures are normally included in the total for the month they occurred in, but some sales are included in the following month's total.