The diagram shows 80" on the inside, with 4"x4" posts, why does it show 90" on the outside?
Well, I couldn't find your exact pergola at newenglandarbors.com, but similar ones did specify a 5" post. So the numbers might be right.

In any case, lay the parts out on the ground. The "bent" that's made up of two posts and the beam that sits between them will probably have a natural way that it goes together. Measure that width and set your base brackets accordingly.