How do I know? Well... they have done it in the past many times. Go read some of the reviews on games with low critic, but high user ratings. Fraud reviews aren't anything new.
Well, a difference between pro reviews and user reviews can have many reasons. The most likely one is that games get updated over time and user reviews react to that while pro reviews are only made at release. Another fact is of course that there is often a much higher user base for user reviews than for pro reviews. And then there is the "herd instinct" in both positive and negative directions which applies on internet users even more than on pro reviewers...
I actually never read a real "fraud" review before which was such an obvious scam. Of course I've read a lot of reviews that I myself thought which were lacking in both writing quality and content but at least the reviewers seemed to get the basic facts rights (which is not the case here)...
I am not saying that user reviews are always correct, but they are often a lot more accurate than the critic reviews. Averages can be strongly adjusted with large deviations. So, it only takes a couple panned reviews to greatly adjust the score.
With the user reviews, it depends on the number. The larger the number, the harder it is for the average to be shifted, which means in order for the user rating to be greatly shifted, you have to have a very large coordinated push to move the score. This happens a lot less than you would think. It is more common in MMO reviews than other games.
There are a few circumstances where a company really blows it on release and then they slowly repair the game over time. Again, this isn't as common as those who receive the terrible scores would like you to believe. Diablo III is a terrible game from its predecessors and it isn't just because of the problems at release. Mass Effect 3 is a horrible RPG and it isn't just because of the ending fiasco. Those games aren't going to repair 5-6 points just because they fixed a single problem.
