Certainly possible--the gulf between the min and max estimates on how many people died in World War II is quite large, and record keeping might be much less prioritized after a nuclear exchange. Also, differences in categorization, even if the figures might not in dispute, could produce massive differences. I could tell you ~20 million died in WW2, or ~70 million did, and both would be acceptable statements--depending on how I defined my terms.
37 million in the initial exchange of our putative World War III is a plausible number, particularly if in the Trek-21stC ballistic missile defense was better (hell, they have genetically engineered super-people, why not a little SDI?

). 600 million could be the figure only when you include the losses from the predicted collapse of the global economy and many nation-states.
Additionally, if we wanted to consolidate World War III and the Eugenics War, maybe Spock's figure (learned from 23d century Earth history texts) refers to a death toll of 37 million
humans, but by the enlightened 24th the 540 million Augments are counted as well.