The number 100 million is special in Japanese. The Japanese don't count as Westerners do, by powers of 1000 (millions, billions, trillions, etc.). Instead, their system for large numbers is based on powers of TEN thousand. So, for example, where we would say one hundred thousand, they would say Ju-man 十万 which is "ten ten thousands".
So one hundred million is actually ten thousand times ten thousand (10 to the eighth power), which is a special number and has its own character 億 "Oku" (actually written 一億 for "one (times) one hundred million"). So in saying 100 million instead of 77 million, they were not being literal, I believe they were invoking the "specialness" of this number as a symbol of the size, strength, unity and specialness/unity of the Japanese population. Kind of cool that they could summarize that in the single character 億 "oku".
They love to do symbolic stuff like this: for example, Japan is often symbolized by the single character for "sun" and the US by the character for "rice". But that is a different pedantic lecture for another day.