To my knowledge Gamefly hasn't really described their algorithm in detail anywhere but I've seen a few patterns. For instance, maybe it's on the top of many people's Q and their priority came a little higher. Or maybe it's number five for someone whose 1-4 are not available at all, while you're 2 was also available. By sending your 2 and his 5 (despite his 5 being your 1) they can maximize utility for everyone.
Have you checked your queue? If it still shows your most recently returned game is out, it may be lost in the mail. If it shows a new game has shipped but you haven't received it, could be same problem in reverse. If games are lost in the mail contact customer service through one of the channels DigitalGuy suggested. If they've received your return, but not shipped a new one, make sure you have games in your queue and are signed up for the correct package.
Not only will they let you know when the game has shipped, they'll also let you know when a game has been received so you don't have to worry about it being lost in the mail.
Occasionally they'll send you emails to check when you received it against when they shipped to make sure their shipping times are okay. Also, depending on your post office, sometimes they'll ship you a game as soon as your envelope is scanned and they know it's coming, but ebfore they even get it. In those cases your account might say you've got an extra game out but it doesn't change your billing.