There's a couple of reasons for using round numbers. An agent or editor is looking at the approximate size of your book. Agents and editors know that if your book is X words, cost of printing will be approximately Y. The difference between 87,345 words and 87,346 is obviously meaningless. The difference between 60k and 80k is significant.
Second is balancing your story against it's size. Depending upon the story (not the genre, the story), an agent or editor is going to know approximately how long the story should be. This is based upon complexity and natural pacing for the story.
Third is whether your book is just too long or short for the genre. Notice that none of these reasons require an exact number.
There's also the question of how accurate word counters are. Word, for example, can count marks such as # (used to indicate an intentional line break) as a word. Bottom line is even if you counted by hand, the chances of an error are so high I doubt anyone ever has a number they like to bet the farm on. I know I don't trust the number to beyond the nearest thousand for a novel. Sometimes it's fun watching it wander around when I'm doing editing, as it tries to recalculate the number.
So there are practical reasons why approximations are used. But the most important one is using an exact number indicates you're an amateur, because people who know use approximations.
Best of luck,
Jim Clark-Dawe
