With electronic media, by the time of World War III there may not have been a single Barnes and Noble still open.
Now combine that with EMP from the wide spread use of nuclear weapons.
Consider this possibility, if he were just referring to combat deaths, his figure for the second world war would seem still much too small.
Now for the longest time people thought the death count for Americans in the Korean war was about 54,000, it was only decades later that there was a study and it was found that the actual number was about 37,000.
When Spock said that there were 11 million (combat) deaths in the second world war, maybe he was right?