There are several criteria for a sale to be credited. Like the likelihood of the client to stay hosting with them should be high enough. Simply put, if the client pays $4 per month and stays for 4 months, is it really fair that they pay you $50 and they make a loss of $34, not to mention server costs? Another thing is that the client's site should be active and used for business, HG won't credit the sale if the site is default installation of wp or has no real content. It translates to them that the client is not intending to build on a business and is not likely to pay the next invoice. The client may also have had an account with them before and is trying to save more by creating a new account to use a coupon. I doubt they shave that hard as people claim, just read the terms and you'll be fine...
Also, common sense. If you get 10 sales and ALL of them are of the least costly package then there is something suspicious about it. Just think about it as a business owner.