When planing a budget, you need to consider how you spend and receive your money vs. how you want to report your budget.
jGnash has to make assumptions when entering per account period amounts. Internally, jGnash is keeping a list of 366 days (365 + 1 leap day) per account with the list starting at the first calendar day of the year.
When a period goal for an account is entered, the amount is averaged across each day of the period. Entry of amounts is also sensitive to the current year. If you select Monthly for the account period, the monthly boundary for days is established by the current year calendar months and the amount is then averaged across the number of days per each month.
Averaging of periods has an impact on how exact the tracking of your budget is. If you choose to enter a monthly average for income, but are paid on certain days on the month, your budget will show slight variations through the year.
If you want the budgeted vs. Remaining amounts to be exact for a particular account, then you will want to set the account period to be Daily and take the effort to enter your daily amount goals.
You will not be able to export a spreadsheet when the report period is daily due to memory requirements and limitations of some spreadsheet applications.