Why?
Because they have their weeks start on Monday and end on Sunday. That Sunday is Sept. 1st, so they included that week under the month of September.
Sigh.. What is with calendars starting their weeks on Mondays? It's confusing! Yea, yea, yea. The first business day starts on Monday. So? Ask anyone the first day of the week and they will tell you it's Sunday. Or I hope they tell you it's Sunday, because it is.
I can't help wonder if these confusing calendars are part of a spiritual agenda. An agenda to validate those who observe the first day of rest vs. those that observe a 7th day of rest.
Is it a stretch? Maybe. But the enemy is always at work. And he is sneaky.