Most people don't write reviews about a game that they hate. If you play a game that you really enjoy, you'll more than likely want to make a review about it to tell people why it's such a great game.
There are plenty of people in this forum alone that have said they don't like Mario 64. Their reviews are most likely not in that data. Most bad reviews about a game is not going to be in the data because most people don't care about wasting their time reviewing a game they dislike.
Not only that, but Discworld has a whole five reviews. Of course there will be some lower reviews to bring down the score. Mario is extremely well known and there are a ton of Nintendo fanboys. Bring in the fact that most people don't review bad games and all the good ratings will pretty much cancel out the bad ratings.
Example. Tactics Ogre had a rating of 1/10, 4/10, 4/10, and 5/10, but because all the other ratings still came out with 8.1/10. Final Fantasy Tactics had similar, but again came out to 8.9/10. Why? These are two well known titles. Then there's Mobile Suit Gundam Encounters in Space(a US released Gundam title) which hasn't had a single bad rating and there's actually a lot of reviews with a 9/10 average, but then check out metacritic with a 6/10 which goes to show you that "legitimate sites" don't know what they're talking about.
Also a 1/10 score on Tactics Ogre at Gamefaqs? Even if you hated the gameplay the story and music were absolutely amazing so the game deserves much more credit than that. I'd say a 5/10 at the lowest if you absolutely hated the gameplay. The game is my favorite game of all time so if I was to write a review about it, it'd be a 10/10. I personally like five ratings better though, as there is no middle number in 10 and people always like to think 5/10 or 6/10 is a bad score when it's actually average.