Depending on how badly the Eugenics and third World War went, I can easily see some information being lost or distorted.
A Colonel Green or Khan Noonien Singh may have had records of certain events modified or even purged for some reason.
Um … are you kidding? Do you have any idea the scope of library and archive destruction that would be needed --- on top of the destruction of archeological artifacts, including
population histories --- to make the number of World War I/II dead that poorly estimated? If one single solitary Barnes and Noble survives the war historians will have plenty of material to get a good estimate from. Heck, a single used bookstore surviving intact would be enough.
At some point it really gets to be easier to suppose that Spock remembered the numbers wrong when he was talking off the top of his head about a subject not one of his primary interests or responsibilities in order to make a rhetorical point in which only the order of magnitude makes the slightest difference.