Why did two pieces of edo-mae sushi become standard?

“In the late 1940s, Japan faced severe food shortages caused by both wartime devastation and restrictions during the American occupation, including a 1947 law that banned restaurants from operating in public. Members of the Tokyo Sushi Association negotiated a compromise with the municipal government that allowed restaurants to open if they relied on a barter system that reflected the spirit of austerity… Customers would bring in a cup of uncooked rice (and pay a very small sum in cash, described as an “ingredient fee”) in exchange for ten pieces of takeout nigiri. Due to regulations on the Japanese ocean fleet, chefs had to rely on a limited selection of river fish and shellfish, and often did not have enough variety to serve ten different toppings. Instead, they doubled up on what was available and gave diners two pieces of each fish. Sushi associations outside Tokyo… usually turned to Edo-style nigiri in place of other sushi forms that could not be proportioned as easily, helping to spread it as Japan’s prevailing sushi form and its serving sizes”

The Sushi Economy pg 70