There is no such thing as a random number when it comes to computers. Every single time you get a "random number" from a computer it is based on a specific algorithm that is meant to simulate randomness.
For most purposes, simply ensuring that over the course of many many many random numbers generated, they average out to the correct number, as that typically indicates a state of "random enough".
The problem is that you will always see patterns emerge from randomly generated numbers. The best algorithms for generating random numbers (currently crypto-based) will create patterns that are VERY difficult to discern without generating billions of numbers, even though they still produce patterns across merely dozens of numbers.
There are a lot of very obvious patterns that emerge out of Blizzard's RNG that suggests they are using old RNG code/techniques and poor seeding practices. And since there are billions of random numbers being generated every single day in this game, patterns are quite easy to see emerge from the end results.