I don't know. The original FireCalc developer is probably the only one who can tell us for sure, and I have not seen a post from him in quite some time.
Just a guess (thinking like a developer, as I was one):
SS always results in a reduction in portfolio withdrawal, so a sign is not needed, but the calculations always use this amount to reduce the withdrawal.
The fields for "Pensions, "Off Chart" Spending Changes, etc." can result in either an increase or a decrease in portfolio withdrawal (depending on which radio button is selected), so a sign is definitely needed for those fields.