Why does the Chicago referencing guide use colons for page numbers in some cases, and commas in others? I love the Chicago Manual of Style notes-bibliography referencing style, although I treat it as the lesser of many evils. For instance, this has always bugged me: why is it that in footnotes, the page numbers of books are preceded by a comma:


  Michael Pollan, The Omnivore’s Dilemma: A Natural History of Four Meals (New York: Penguin, 2006), 99–100.


But the page numbers of journal articles are preceded by a colon?


  Joshua I. Weinstein, “The Market in Plato’s Republic,” Classical Philology 104 (2009): 440.


I like the colon, since it clearly shows that the next section of the reference is from within the reference (i.e. it is a specific page number). But I'm going to get raised eyebrows if I use colons everywhere in my footnotes. So why the discrepancy in the first place?
The difference in the format helps differentiate articles from books.

Each citation format (Chicago, MLA, IEEE, APA, etc.) have their own way of differentiating articles and journals. This is just the way it is done in the Chicago style.