Except that sometimes you land plop in the middle of the Unpublishable Void -- or near enough to its brink that fiddling with length is wise. Putting the story in the cheese press to squeeze out the extraneous and get it under -- one hopes -- 8000 words. Or trying to weave in a few more strands to get it over 75,000 -- or 1000, since some markets don't take shorter than that.
The real annoyance is when you hit a spot where (even though you're shortening) it lacks clarity or liveliness or any other necessary virtue that will need more words, or (even though you're lengthening) it has conspicuous deadwood that needs hacking out. So you sigh and brace yourself and do the damage to the length you want trying to tell yourself that better story is better than right length. . . .