· Overcrowding. Your book may be unique relative to the overall market, but if an account's buyer has already bought five other titles that are (to him or her) essentially identical, their buy for your book is going to be lower than it otherwise would have been—doubly so if your title is designed to appeal to a very small or niche audience. (This is one reason why trend chasing can be a bad idea.) You have no control over what other people are writing, agents are pitching, editors are acquiring, and sales reps are selling, so there's no real way for you to account for this ahead of time. You'll just need to write what you want to write as well as possible.
· Publicity. Sure, you have control over your reputation as a person and a writer, but the kind of publicity I'm talking about is the pie-in-the-sky stuff about which even the publishing house can't make any guarantees: an Oprah's Book Club seal, a spot on Good Morning America, &c, &c. These are game-changing factors that will certainly boost sales of your book, but you can neither account for nor depend on them beforehand.
· The sales call. Maybe the rep isn't super enthusiastic about your book (though he or she will do his/her absolute best to seem so). Maybe the buyer doesn't like the title of your book, or hates the cover, or is simply having a bad day. Maybe the sales call is running over and the rep only has thirty seconds to sell your book. There are innumerable factors that affect the actual sales call over which you have no control whatsoever, and each of them can influence the account's buy one way or the other.
· Plain old luck. I've said it before and I'll say it again: good luck or bad, it's going to affect your career in some capacity. Maybe you got that Oprah's Book Club pick! Maybe you've had to change editors six times since acquisition. Maybe you wrote that killer bio of Millard Fillmore right before they discovered he was secretly a space Nazi!* Maybe your book gave Stephen King a terrible papercut, prompting him to write a scathing public review. The list is endless.
The point of all this, author-acquaintances™? Many aspects of your career will be difficult to predict or control. Everything from having your partial MS passed on to the agent by his/her assistant to the final sales call to winning all kinds of fancy literary awards will inherently contain an element of randomness or luck, and it's up to you to make the best of it, regardless of the circumstance.
*The author has no reason to believe Millard Fillmore is, was, or ever has been a space Nazi. Sorry, Millard.