(In). In whole numbers, without regarding the fractions. Thus we
say the population of the British Isles is forty millions in
round numbers, and that of London four millions (1895). The idea
is that what is round is whole or perfect, and, of course,
fractions, being broken numbers, cannot belong thereto. Source:
Dictionary of Phrase and Fable, E. Cobham Brewer, 1894 More on
Round Numbers from Infoplease: