They probably eyeballed the amount of food laid out, looked at the number of guests, and thought there was guaranteed more servings of everything, because otherwise there'd only be enough for like a 1/4 plate of food per person.

I really doubt they managed to pile up 4 plates worth of food onto one, though. It sounds like the rations were planned out for basically 1/4 of a plate of food per guest (1/2 plate if we're being generous), and the guests just took normal or a bit over.