Have you ever noticed? September is called the 9th month on our "normal" pagan Gregorian calendar, but "Septe" means 7! October is the 10th month but "Octo" means 8. November is called the 11th month but "Nove" is 9. And, of course, December is the 12th month but "Dece" means 10!
Don't you see how Yehovah's calendar is the original and we have simply gotten away from it by following man, instead of God??