The double unary is used to coerce the Booleans to their Integer equivalents.

In the OR by means of addition example (as opposed to Match) you will note we do not use the double unary - this is because the act of addition will coerce the Booleans thus double unary not required.

For a more detailed overview of SUMPRODUCT and indeed coercion see the link in my sig. to Bob Phillips' white paper.