Dear Youdont,
Maybe you don’t see it, but 600,000 is the only number that is real according to Rav Moshe. For a city, that is the amount that is out and about daily on the streets. That is what he wrote in the first teshuva. He reiterates it in the second teshuva. And he points out that it is the foundational point in the third teshuva.
All the other number are ways to come to 600,000.
And Rav Moshe writes that the other numbers are not to be relied on. Not because of a gezeira. Because those are not real numbers.