Updating Variables

Hello, I have a question that I do not understand why I have to answer. annual rain (from September) if I only ask for it in September and December, but if I put it, it puts me wrong, and I do not understand why I have to put it every month.
Could someone explain it to me?

september_rainfall = 5.16

annual_rainfall += september_rainfall

october_rainfall = 7.20

annual_rainfall += october_rainfall

november_rainfall = 5.06

annual_rainfall += november_rainfall

december_rainfall = 4.06

annual_rainfall += december_rainfall


that’s not the correct link. please provide the correct link so we can see the instructions, etc.