net income = 0 part still unclear for me

Hi,

I have searched the question section and I already understood the theory part about if net income = 0 no error has been made previously.

I also understand how you get the net income by taking away the various costs and taxes...

But what I do not understand is how when I apply a "-" (minus sign) to the formula I do not get 0??? (I downloaded the "sumif_before" ressource and retried doing the exercise myself)

I think my question would be, what item needs to be equal to 0 so that I am sure that I haven't made a mistake in my work since checking that everything is correct is an important step of this exercise.