Well think about it, even if it's 100% even distribution, the number of games on Steam that you are not interested in far exceeds the number of games that you are. The number of games you are interested in but already have is also probably much larger than the number you don't have and want. So statistically you're far more likely to get a coupon for a game you're not interested in than one you are, and you're also more likely to get a coupon for a game you already have. And that's assuming that they haven't played with the probabilities at all.