This happened to me a couple years ago and ever since, I've been jotting down the word count when I start a new chapter or after I've added a particularly hard-fought paragraph to my manuscript. See that photo above? Those are my numbers from last week. Looks kind of crazy, doesn't it? Sort of like this?:
Anyways, since I've been manually keeping track of my numbers, I've been able to notice a weird quirk in Microsoft Word: the software is actually pretty shitty at calculating word counts, or at least remaining consistent in how it performs the calculation.
Here's what I mean. Almost every time I return to my manuscript (after I've rebooted my computer) I find that the word count -- which I compulsively recorded on my notepad -- has changed, in either the plus or the minus, depending.
When I first noticed the problem, I would get very paranoid. "Did I hit a few keys by mistake?" "Did I delete something without noticing?" "Oh $#!%, oh $#!%, oh $#!%!!" and I'd re-read entire sections looking for my error. But I never found one.
What I did find, however, was this very valuable lesson: While you should always trust your words, dear writers, never ever completely trust your Word.