The official website for The Settlers of Catan

Question about Catan - Cities & Knights for PC/MAC Do the dice rolls really produce random results?

Answer

Particularly when things are not going well and the robber comes exactly in the worst moment, players are sometimes quick to allege that the random number generator is flawed or manipulated. However, it has been proven in extensive tests that this is definitely not the case.
Because two dice are used, the numbers rolled in each game basically follow a triangular distribution, meaning that, for example, a “12” occurs less frequently than a “6.”  One has to keep in mind, however, that we are dealing with statistical probabilities. It is therefore possible – although not very likely – that a “7” is rolled 7 times in a row. In a normal Catan game with approximately 60 dice rolls in total, the number of rolls is too small to always result in a perfect Gaussian distribution. Certain anomalies may well occur, and it could happen that the “6” is rolled more often than the “8,” even though the expected probability for these numbers to be rolled is the same. But all this doesn’t mean that the random number generator is flawed or manipulated – it’s simply in the nature of things and wouldn’t be different when you play at the table.
In order to make the dice roll results as random as possible, the routine used for the random number generator is the renowned Mersenne Twister method. Among experts, this routine is considered as the ideal combination of speed and performance (in this case, randomness)
More information about the method can be found here.