You're looking at it backwards. If you're using Courier 12 pt. with 1" margins, then 78 pages times 250 wpp is 19,500 words. That's your word count using traditional counting methods, not 15,000, which I'm guessing is your word processor's count. Word processor counts won't match 250 wpp x nbr. of pages.
I understand what you mean, but I also understand why the OP asked the question. People are constantly claiming that 250 wpp. is a pretty good estimate of your word count--and for fiction, it generally isn't; fiction is all over the place. On a single page of terse dialogue, word counts can plunge toward nothingness.
I thiink Dragoon Elf is asking if his word density over his 78 is low. I'd say it's on the lower side of the normal range. Nothing to worry about.
Knowing your word count and your typical word density can be useful. And the writers I know put a rounded count of actual words on their manuscripts, not 250 x pages.
Even before the advent of word processors, writers used to count their words. In Twain's original manuscripts, there are little numbers pencilled in the margins--counting words. Hemingway and Graham Greene did the ame thing. I certainly count my words. And, because I've been writing long enough to know what's typical for me, I pay attention to things like word density per chapter in the same way that most people give at least passing consideration to chapter length.
Just as a matter of trivia, I'm told that in France, publishers are interested in letter counts--all characters including spaces. They scoff at the notion of adding together "a" and "sesquipedalian" and claiming that it measures something significant.
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