A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
ALL of the annuity functions work that way. From HELP:
In annuity functions, cash you pay out, such as a deposit to savings, is represented by a negative number; cash you receive, such as a dividend check, is represented by a positive number.
In this case, since you are paying money to the bank, it is represented by a negative number.
You can reverse the convention; the important concept is to be consistent.