The period switcher shows the same recurring scenario as a day, week, month or year. It does not change the items you entered.
Recurring frequencies are converted across a full year. For example, weekly money is based on 52 weeks and fortnightly money on 26 fortnights. Monthly figures are then based on the yearly total divided by 12.
Small differences from multiplying a week by four are normal. Four weeks is only 28 days, not an average month.
Tip: Choose the period that makes the decision easiest to understand. Monthly is useful for the whole budget, while weekly can make a small change feel more concrete.