Well, once could assume that the numbers you see are in the millions, they are just shortened for easier viewing.
( Population = 322 = 322000000 people paying tax/being citizens of your empire)
Considering that Sins is game on an interstellar scale were huge empires fight each others, there is no reason for you to know if a planet has a population of 321.994994 or 321997943 people. 321 is all you need to know as a few more or less tax payers are of no consequence at this scale.So it is just for the easier viewing
I dont think that those are individuals, as an average fleet has a crew counts that goes into the 100000.