WHYdoes my calculator keep doing that?

I use a financial calculator (with the "five magic keys!") to do my time value of money calculations.

Every time I enter a set of variables, my calculator keepschanging the sign!If I put in positive values, it gives me a negative answer.If I put in a negative number, it gives me a positive one.

If I use the equations, it doesn't do that.

If I use the tables (but,why would I?) it doesn't do that.

But only if I use a calculator (or excel) does it do that.Why?