>there's more than one way to do math

There are ways that give you more and less accurate results.

If you take a week worth of data and multiply to get a month, you're also multiplying any mistakes or outliers there might be. If you take 3 months worth of data and dividing to get a picture of your average month, you're dividing, i.e. ironing out any outliers. That gives you better predictive power to predict, say, how much money you're going to need in the next 10 months.

Say you eat at home almost every day of the month, but once per month you eat at a fancy restaurant. If you take data from the week you ate at that fancy restaurant and multiply by 4.345, you're going to make it seem like you eat at a restaurant 4 times per month. If you take data from the non fancy restaurant week, your fancy meal won't be captured in the data at all. Either way, you're getting inaccurate monthly data.

Longer time frame = more accurate result in terms of predictive power (e.g. predicting how much money you're going to need over the, say, next 10 months)