The displayed available amount of a Budget changes depending on the selected time period, i.e. if a certain month is selected, then for that month, and if a year is selected, then for that year. For this reason, the amounts are calculated according to the selected period. The reason for this is that you can see, for example, whether you are within the monthly limit for an annual Budget.
If you want to see the available Budget for this month, simply change the period to the current month using the “Year” button at the top left of the Budget page. Similarly, if you want to extrapolate a monthly Budget to a full year, you can select the current year in the period selection view. Additionally, you can also select a custom time period (e.g. next two weeks) to calculate the Budget for that time period.