Back in the good old typewriter days, you basically had a choice of two kinds of machine when it came to the look on a page: Pica, and Elite. The difference was in characters per inch: Pica gave you ten, Elite, twelve. There were other typefaces besides "typewriter," which is pretty close to courier font, but nobody used them to write fiction -- that was a fast way to get your manuscript bounced.
I was always a Pica man.
Estimating word counts on a manuscript back then was also simple. You'd take three or four pages out of a ms, do a total word count, then divide that by the number of pages. You didn't make exceptions for dialog or dense exposition, you just applied the number straight across.
A page of dialog has fewer words; however, they don't stuff three pages of dialog onto one to make the word count work -- the white space is part of the dialog. In the days when printers charged by the page, it didn't matter how many words were on sheet, the paper cost the same.
For most of my writing career, my manuscripts, with one-inch margins all the way around, I got 250 words per page. Made it easy to figure, and over the length of a novel, was pretty accurate. Four hundred pp? 100,000 words.
Then the computer thingees came along, and as they got more complicated, why, they figured it out for you. Five letters and a space = one "word." (Words like "if," "and," or "but," didn't count until you strung them together.
I'm not sure this is progress, but that's how it is.
On a typewriter, once you set your margins so you average ten words a line and twenty-five lines to a page, you never had to worry about it changing. Long as nobody else fiddled with your machine, you were golden. (And never once did my typewriter burp and tell me it couldn't find that page, that no such file existed.)
Computer programs, however, sometimes do funny things when you aren't looking ...
The book-in-progress has been swimming right along, flowing quickly. The pages have been adding up, I can see a running count, right up there in the corner. But since I use a 150% view on my word processor, so as to better, you know, see things, I don't get a whole page of text and the rulers on the screen at once, and I didn't realize that I -- or the gremlins that live in my system -- had somehow jiggered the margins on my novel template. So instead of getting 250/words/page, I was getting closer to 200/w/p.
That it was flowing so fast should have been a clue, but, alas, wasn't. So as I looked down and noticed that my first draft was almost done, and over 350 pp, I was pleased, since I was looking to bring the novel around 85,000 words, and that would give me a little room to trim and tighten things
So I ran a word count, to see how close I was ...
10,000 words short, that's how close.
Oh. That's no good at all.
So I opened the file at 100% view, saw right away that the body of the page was short, top and bottom, and, oh, my ...
Not a problem, in that I can go back and add new material in; it's just more work to do it that way than it is to cut it.
Lots easier to make a rope shorter than it is longer.
Do not trust your computer. It's waiting for a chance to get you.